Authorities here and in West Germany are investigating whether Syrian diplomats were involved in the unsuccessful April 17 attempt to place a bomb aboard an Israeli airliner at London's Heathrow Airport and in the March 29 bombing of the German-Arab Friendship Society in West Berlin.

In a terse statement today to Parliament, British Foreign Secretary Geoffrey Howe confirmed that discussions had been held with Syrian Ambassador Loutof Haydar during which "we discussed the El Al bombing." Howe said he could "give no other details."

In West Berlin, justice officials said that two Arabs who have confessed to the March 29 bombing of the German-Arab Friendship Society there have told police that they smuggled the explosives into the West from the Syrian Embassy in East Berlin.

The two, Ahmed Nawaf Mansour Hasi, a stateless Palestinian, and Farouk Salameh, a Jordanian, originally were arrested for complicity in the separate bombing on April 5 of the La Belle discotheque in West Berlin that killed two persons and injured 230. Police said both have continued to deny involvement in the disco bombing, which induced the Reagan administration to launch retaliatory air strikes against Libya for its alleged support of terrorist activities in Europe.

The new reports of possible Syrian involvement in such activities raised anew the question of whether the West would be prepared to take similar military or other action against Damascus.

In an interview last month, President Reagan said that he would use military force against Syria or Iran if presented with clear evidence that one of those governments had sponsored an act of terrorism against Americans. Other western governments, however, have avoided direct accusations of Syria.

A statement Monday by the seven governments at the economic summit in Tokyo pledged them to apply a series of sanctions "in respect of any state which is clearly involved in sponsoring or supporting international terrorism."

The statement mentioned only Libya by name.

Reagan, pressed in a news conference yesterday as to whether the statement was "a warning to the Syrian government as well as to the Libyan government," said it was directed toward "those countries which there's reason to suspect have, if not openly supported, certainly not discouraged, terrorism."

One western diplomat here voiced the dilemma, saying that "it's one thing to mess around with Libya," a fairly isolated country whose leader, Col. Muammar Qaddafi, is distrusted and disliked even by many of his fellow Arabs.

But Syria, the diplomat said, "is a serious country," allied with the Soviets and a major player in the Middle East, and the ramifications of a military strike or significant sanctions could be much greater.

In a statement today in East Berlin, the Syrian Embassy vehemently denied complicity in the two West Berlin bomb blasts, which police say they believe involved the same type of explosives.

An embassy spokesman, Mohammed Bassam Imadi, said Syria was "opposed to any kind of terrorism and had nothing to do with any explosion whatsoever that occurred in West Berlin." He charged that the confessions elicited from Hasi and Salameh were "lies being spread in order to damage Syria's reputation in the world."

Allied intelligence sources also expressed caution about the testimony of Hasi and Salameh. "They've told different stories already, and we are still sorting all this stuff out," one official said. "They are obviously trying to save their own skin."

U.S. officials in West Germany refused to comment on whether Soviet and East German authorities were being approached to enforce action against the Syrian diplomatic mission and determine whether members of its staff in East Berlin kept contact with the suspects.

West German security officials said that while independent evidence has confirmed that Hasi went to Libya recently, the Syrian link still is being treated judiciously.

Western diplomats have said there is growing concern about the possible ramifications of connections between the West Berlin and London bombing investigations. Nezar Hindawi, a Jordanian citizen arrested in the London case, is the brother of Hasi.

Hindawi, 31, was arrested here on April 18, the day after he allegedly gave his unsuspecting girlfriend a suitcase bearing hidden explosives just before she was to board the Tel Aviv-bound plane.

Scotland Yard notified West German police that Hindawi's brother was living in West Berlin, a tip that quickly led to Hasi's arrest. After being threatened with deportation, Hasi reportedly has been speaking freely to West German police.

While denying involvement in the La Belle bombing, he reportedly has said that he and his brother, who visited him in West Berlin in February, together planned the bombing at the friendship society, enlisting the services of Salameh and a fourth Arab, Fayez Sahawnah.

Police here have said that Hindawi, who has been charged with conspiracy to blow up the El Al plane and murder its passengers, entered Britain illegally on a false Syrian passport. But British authorities have been more circumspect than their West German counterparts in pointing a finger at Syria. Sources said that Haydar, the Syrian ambassador, was called to the Foreign Office late last week and asked for Syrian government permission to interview "more than one" diplomat in connection with the case against Hindawi. At least one of the Syrian diplomats sought is believed to have left Britain.

On Monday, Haydar returned to the Foreign Office with an initial response from Damascus that one source said constituted the beginning of "negotiations" over "a messy situation" involving possible claims of diplomatic immunity and the risk that publicity about the investigation will damage the case against Hindawi in British courts.

Officials here are known to be angry over foreign government statements and press reports directly accusing Syria of involvement in terrorism. Among others, they referred specifically to comments made yesterday in Washington, where Vice President Bush said, "We are convinced that Syria's fingerprints have been on international terrorist acts."

Bush's statement came after a White House meeting with Israeli Defense Minister Yitzak Rabin. Rabin was even more specific, saying that Israel had evidence of Syrian complicity in the El Al bombing attempt. A report in Monday's Jerusalem Post, sharply denied by officials here, said Britain was preparing to expel at least one Syrian diplomat in the case.

British courts are extremely sensitive about any publicity surrounding a case still before the courts. If public commentary persists in saying Hindawi "did it, and had Syrian backing, the trial will be called off before it even starts," one official here said.

During Hindawi's two court appearances thus far, his attorney has made repeated references to "biased" reporting and statements about the case. He has blamed police for leaking information.

At the same time, Britain is nervous about "negotiating" with the Syrians any arrangement that will recognize the right to diplomatic immunity and a withholding of publicity in exchange for information that might help convict Hindawi.

Prime Minister Margaret Thatcher's government is still smarting from domestic criticism it received two years ago after a British policewoman was killed by a shot fired from within the Libyan embassy here.

After 10 days of tense negotiations, 30 Libyans inside the embassy in London -- one of whom presumably fired the fatal shot, and only 19 of whom turned out to be accredited diplomats -- were allowed to leave the country.

Washington Post correspondent William Drozdiak contributed to this report from Bonn.