DAMASCUS, SYRIA, SEPT. 19 -- Syrian President Hafez Assad put an end to a year of open diplomatic conflict with the United States today by welcoming recent decisions by the Reagan administration to lift most of the sanctions imposed against this strategic Arab nation in a dispute over terrorism.

In a two-hour interview at the presidential palace, the authoritarian Syrian ruler adopted an unusually conciliatory tone in his remarks toward U.S. policies in the region. But he indicated that important differences remain, principally over U.S. support for Israel.

Assad also for the first time confirmed that he had met secretly in April with his most bitter foe in the Arab world, Iraqi President Saddam Hussein. He disclosed that no progress had been made in ending their decade-old conflict, noting, "We each had our own view on Arab and international matters. . . . We remained where we were."

Emphasizing that he had no intention of reducing Syria's political support for Iran in its war against Iraq, as has been frequently rumored, Assad asserted that Syria's refusal to break with Iran "has played a major role in preventing the expansion of the war" to other Arab states.

Assad, 56, appeared today to have recovered substantially from a series of health problems that had weakened him in recent years. He was far more animated in conversation and physically relaxed than he was during a similar interview 16 months ago, when he openly voiced fears that the United States was considering a military attack on Syria.

Such concerns were absent from today's interview, which Assad used to make these other points: Syria continues to seek strategic military parity with Israel despite a lull in military tensions between the two countries and indications by the Soviet Union, Syria's main military supplier, that it is seeking to reduce confrontation with Israel.

"It is difficult to see that Israel would respond to the need for a just peace while it is feeling superior," Assad argued, adding, "Those who want peace to prevail, whether in the East or the West, should view our sayings and our direction as a fair saying and direction, and a necessity." Syria will continue efforts to help free more of the foreign hostages held in Lebanon. "We approach the subject of the hostages as a human issue," he said. "We understand their suffering."

But he could offer no specific indication that any new releases could be expected soon. He confirmed for the first time that Syria had closed the Damascus office of the Palestinian guerrilla group led by Abu Nidal, which has been implicated in the slaughter of civilians at the Rome, Vienna and Karachi airports last year.

But he strongly rejected any implication that he closed the office to restore normal relations with the United States.

"It is wrong to portray the action taken with the presence of the Abu Nidal group as if it was taken under the influence of a foreign power, because the actions taken were for reasons belonging to us in Syria," he said. He did not detail those reasons.

U.S. officials cited the closure of the Abu Nidal office as the primary factor in the decision to return Ambassador William L. Eagleton Jr. to his post in Damascus on Sept. 2 and to remove barriers to U.S. oil companies operating in Syria.

Washington imposed these sanctions last autumn after Britain formally named Syrian officials who allegedly helped stage an unsuccessful attempt to plant a bomb aboard an Israeli El Al airliner at Heathrow Airport. Britain broke diplomatic ties with Damascus as a result of this incident.

Disputes over terrorism, U.S. support for Israel and Syria's role in Lebanon have kept tensions high between Washington and Damascus for nearly four years. They exploded into military action in 1983 when two American fighter-bombers attacking Syrian positions in Lebanon were shot down and a U.S. Navy flier taken prisoner by Syria.

The U.S. bombing of Libya in April 1986 and the increasingly pointed accusations leveled by the United States and Israel about Syrian involvement in terrorism at the same time produced a new crisis atmosphere that continued until this spring, when U.S. officials said they became convinced that Assad had moved decisively to prevent Syria from being used as a planning or staging ground for terror operations.

Against this background, Assad's carefully calibrated and restrained phrases today, welcoming the renewal of a dialogue with Washington through the return of the ambassador, appeared calculated to suggest that he sees a new opportunity for Syrian-American relations.

Assad, whose remarks were interpreted by a palace official from Arabic into English as he spoke, sipped a single glass of fruit juice and a cup of Turkish coffee during the lengthy conversation.

The removal of the most important American sanctions also coincides with Syria's hosting this month of the Mediterranean Olympic Games in the port city of Latakia, where the Syrian leader spent most of this week.

In contrast to the besieged image he projected in the 1986 interview, his mood today was buoyant. His confident demeanor was that of a leader who has reestablished unchallenged political authority at home and now broken out of diplomatic isolation abroad.

He also optimistically predicted that Syria was gradually overcoming the severe economic problems of recent years with the help of new petroleum, diamond and other mineral discoveries.

An Air Force general when he and other military officers belonging to the Baath (Renaissance) Arab Socialist Party seized power in 1970, Assad has imposed a strong grip on this once turbulent country, using multiple internal intelligence agencies and dividing the power of his military commanders to prevent the kind of intrigues that toppled his predecessors.

Under Assad, Syria has projected an image of being the most adamant Arab foe of Israel and has amassed a powerful arsenal of sophisticated weapons supplied by the Soviet Union. Syria has been widely regarded in recent years as the Soviet Union's closest ally in the Middle East.

He also has fashioned a political alliance with Iran in reaction to his bitter feud with Saddam Hussein of Iraq and has made Syria the only important Arab country to support non-Arab Iran in the seven-year-old gulf war.

Assad's repeated emphasis today on the unchanging nature of Syria's policies and tactics in all of these regards was consistent with his effort to project his power as solid and unbending under pressure.

But with the clear exception of his determination to be ready to fight Israel militarily if necessary, his comments also suggested that there is some fluidity in Syria's intricate patterns of alliances and confrontations.

Assad went to some lengths, for example, to avoid blaming the Reagan administration for the diplomatic conflict of the past year, saying that Israel had manufactured the crisis. He offered no substantiation for this accusation.

"There is no serious and direct reason which would have caused the deterioration in relations" between the United States and Syria, he said.

In the past, Assad has been an extremely harsh critic of any U.S. military presence in the region. But in answering questions today about his reaction to the military build-up under way in the Persian Gulf, he failed to single out the United States for the kind of denunciations that have been repeatedly voiced by Iran.

"The presence of the fleets in the gulf creates additional dangers with unpredictable results," he said in response to one question, omitting the demands that the Soviet Union and Iran have made for the withdrawal of all U.S. warships from the gulf as a way of reducing tensions.

But Assad predicted that "it will not be possible to reach any results through the use of power or pressures" on Iran. Instead, he said, "it is possible to reach results with Iran through friendly dialogue."

Speaking about the lack of hostilities along the military lines of the Golan Heights and portions of southeastern Lebanon controlled by Syrian and Israeli forces, Assad indicated he did not believe that this represented a long-term change.

"At present, we do not know what is going on in the minds of the Israelis. . . . Israeli expansion, we believe, is a strategic goal, not a tactical one," he said.

He declined to discuss directly whether changes in the Soviet Union's Middle East policies were affecting Syrian-Soviet relations. Under Communist Party leader Mikhail Gorbachev, the Soviets have inaugurated a new political dialogue with Israel and emphasized that they will supply only defensive weapons to Syria.

Assad insisted that Soviet shifts would not affect Syria's declared objective of obtaining "strategic parity," or matching Israel militarily, a goal that now appears increasingly distant to trained observers here.

"Strategic parity should be achieved, and there is no change or rethinking of our attitude in this regard," Assad said. "Any country that desires a just peace in this region should view the strategic parity that we are seeking as a fundamental, positive factor in achieving peace."

The Syrian president expressed no opinion on the impact of the violent demonstrations that led to the deaths of at least 400 people in Mecca, Saudi Arabia, July 31 other than to reaffirm Syria's opposition to calls in the Arab world to break relations with Iran.

"After Mecca, some Arabs wanted to change relations with Iran and some changes were discussed," Assad said. He added, "It is against the interests of the Arabs to break relations with Iran." Arab League foreign ministers are due to meet in Tunisia Sunday to consider demands by Saudi Arabia that Arab states break off relations with Iran. Assad, indicating that Syria will fight such a move, noted that not even Iraq has broken diplomatic relations with Iran.

In Lebanon, a number of western and Arab leaders see Assad's forces facing an increasing threat from the fundamentalist Shiite Moslem militia Hezbollah (Party of God), which is aligned with Iran and resists Syrian authority. Assad indicated that the Iranian-inspired presence is only one part of his problem.

"It is very difficult to ascribe responsibility for all the violence to Hezbollah," he said. "The responsibility of Hezbollah in internal violence is less than the others."

Asked whether the 7,000 Syrian troops deployed to west Beirut last winter seek to contain Hezbollah in its stronghold inside Beirut's southern suburbs, Assad said such an idea was "unrealistic."

"We are seeking to spread security in Lebanon through reconciliation. . . . We are trying to have the civil war not return to the hot stage it was."

Assad said next year's presidential elections in Lebanon would be a symbol that the country -- now badly divided by competing power groups -- retains a national identity.

"The presence of a single president is a symbol of one Lebanon," Assad said, adding, "There are multiple authorities in Lebanon and at the same time one Lebanon."

On efforts to arrange an international peace conference involving the parties to the Middle East conflict, Assad declined to elaborate on remarks attributed to him by former president Jimmy Carter, who reported last spring that Assad was ready to enter into direct negotiations with Israel after such a conference is convened.

"When there is unanimous agreement on convening the conference . . . then we will move to a discussion of the procedures," Assad said.

Assad's assertions today that he had acted on his own in closing down the Abu Nidal office here were echoed in a series of background discussions with senior Syrian officials and diplomats this week.

They portrayed Assad as determined to resist any hint of "knuckling under" -- as one western diplomat put it -- to U.S. pressure on terrorism.

"When we move out the offices of Abu Nidal," said a senior Syrian official, "this is for national interest and not for this country or that country.

"From the beginning, we said Abu Nidal has an office in Damascus, but we in Syria didn't allow this office to be involved in any terrorist acts," the official said. "But the Middle East is so complicated, why should we be responsible for Abu Nidal if he is making hijackings {in other countries} because he has so many links abroad."

One western official said the United States presented information to the Syrians showing that the Abu Nidal group had used Damascus airport and other facilities in Syria to support terrorist attacks against civilians in other countries.

Evidence gathered after a September 1986 attack on a Pan Am jumbo passenger jet in Pakistan was part of the American case, this official said.

"The Syrians were upset," the western official said. "Abu Nidal had this operation, and it got traced back to Syria to the Abu Nidal presence, and we used it to enlighten the Syrians about what was going on, and that was a good reason for them to tighten up."

A senior diplomatic source said that movement by the Abu Nidal group to mend its fences with Yasser Arafat, head of the Palestine Liberation Organization and an archenemy of Assad, had helped trigger the Syrian decision to close down the group's operation here.