Leaders of the seven-nation economic summit that opened here yesterday neared agreement on a pair of declarations on state-sponsored terrorism and the Soviet nuclear accident, the tone of which "extremely pleased" President Reagan, White House spokesman Larry Speakes said today.

"We believe it is a significant step forward," Speakes said of the terrorism statement. "It does not restrict unilateral action and it provides for coordination of actions among various nations in order to combat terrorism."

Representatives of the seven national leaders worked through the night here to produce draft agreements that were expected to be announced by Japanese Foreign Minister Shintaro Abe later today, assuming that the heads of state and government give their final approval.

Despite the expected formal expression of unity, sources from several countries represented here said that serious differences had existed yesterday on the content of a proposed antiterrorism statement.

British sources said today that the French and Japanese still had some "legal questions" about key portions of the terrorism document.

European officials said the key element of the statement on the Soviet nuclear accident calls for an international convention to make mandatory the steps that are now just guidelines of the International Atomic Energy Agency for reporting nuclear accidents.

The awaited twin announcements would dramatize the degree to which this 12th annual three-day economic summit has been overshadowed by the issues of state-sponsored terrorism and the Soviet nuclear accident at Chernobyl.

In another development here, British officials confirmed reports that the new Soviet ambassador to Britain, Leonid Zamyatin, recently had presented to Prime Minister Margaret Thatcher a letter from Soviet leader Mikhail Gorbachev dealing with East-West relations. British sources said that the letter and the ensuing conversation with Zamyatin had been interpreted by Thatcher as a signal that Gorbachev is still interested in a U.S.-Soviet summit this year. The letter was written before the Chernobyl accident.

Speakes said that the United States did not know what to make of this, and his deputy, Edward P. Djerejian, said, "We have never got any indication through any private channels" that Gorbachev would not come to the United States in 1986.

Prior to today's comments by Speakes on the terrorism issue, a senior U.S. official had said last night that West Germany and Italy were "leery" of a tough statement calling for specific measures. He said the French were historically against any separate statements on political issues but "would probably go along with pap," meaning a relatively bland statement, in the interest of unity.

But even if the public statement fails to go much beyond the one drafted in opposition to terrorism at the London economic summit in 1984, U.S. officials are counting on the allies to agree privately to specific but unannounced steps that would keep the pressure on Libya, the prime target of the U.S. campaign against state-supported terrorism.

Officials said that the Europeans are much more inclined to go along with actions that are not formal, public commitments. "I would rather have each country agree to do something and keep it quiet until they actually do whatever it is they're going to do," White House Chief of Staff Donald T. Regan said on the NBC television program "Meet the Press" yesterday.

Among specific measures mentioned by Speakes yesterday were closing of embassies, denying airline landing rights and halting communications with nations that support terrorism. A French official said that the proposal discussed last night would ban arms sales to nations that harbor terrorists and improve procedures for extraditing them.

In an effort to soften European criticism that the United States has been asking others to do what it is unwilling to do itself, Regan said, "We have set a deadline for American oil companies to get out" of Libya. A senior U.S. official said later in the day that the American companies will be required to withdraw in June.

Regan stressed the importance of specific actions against terrorism as the test of allied sincerity.

"I think that everybody can abhor terrorism, can decry terrorism, can say that terrorism is a bad thing," Regan said. "But I think now it's time for the civilized world to cut this cancer out of its body . . . . Talking about it is not going to be enough. We need action."

A British spokesman said that the statement on terrorism may appear to be aimed at Libya but probably won't mention Libya directly.

Omitting specific mention of Libya may make it easier to win the support of Italy, a leading Libyan oil customer and trading partner. On Saturday, Italian Prime Minister Bettino Craxi reiterated to Reagan his government's persistent opposition to economic sanctions against Libya. But Speakes said that Craxi also told Reagan that there are steps that could be taken and that the number of Italians in Libya will be reduced greatly.

West German Chancellor Helmut Kohl also restated his opposition to economic sanctions in his meeting with Reagan yesterday, according to a senior U.S. official.

Appearing on the CBS television's "Face the Nation," Treasury Secretary James A. Baker III said that many leaders at the summit "say economic sanctions don't work" because they have never tried them.

"If everyone were willing to jump in there and exercise or invoke economic sanctions, they would work and work well," Baker said.

U.S. and British sources said there was no discussion of the U.S. April 15 bombing raid of Libya at a working dinner last night. Nor was there discussion of the rocket attack that marred the day's opening ceremonies at Akasaka Palace.

The British were described by a U.S. official as "leading the charge" for the terrorism statement, as Thatcher did two years ago.

A British spokesman said that a statement was drafted in broad form by the Japanese and that the British then responded with their version. A French official said that the British text became the centerpiece of the discussion at the dinner last night. Canada and the United States are expected to support the British draft.

On the nuclear issue, the leaders appeared to be moving toward a statement that would emphasize safety and better international sharing of information after nuclear accidents rather than one that would criticize the Soviet Union harshly.

But U.S. officials continued to blame the Soviets for failing to provide adequate information about the nuclear accident at Chernobyl. Regan said in his television appearance, "Frankly, the way they've handled it is an outrage."

A British official said that the nuclear statement probably would express sympathy to victims, offer assistance and request all countries to honor their obligations in case of an accident. The official said it may also seek to set basic "standards" for nuclear power plant operation and maintenance.

Friedhelm Ost, spokesman for Kohl, said that Reagan in his talks with the West German chancellor had supported Bonn's call for improved nuclear safety standards and faster notification of accidents. The West German aim is to reduce sharply the present 40-day notification period for nuclear accidents set by the International Atomic Energy Agency, based in Vienna.

The Soviets have cited this 40-day rule in rebuffing criticism by western nations that they delayed too long in giving an account of what happened at Chernobyl.

Both U.S. and European officials are trying to prevent public concern over the Chernobyl incident from increasing opposition to nuclear power in the West. Reagan and other top U.S. officials repeatedly have emphasized the safety record of U.S. nuclear plants.

This afternoon the summit members issued a political declaration entitled, "Looking forward to a better future," a reaffirmation of the seven nations' commitment to democracy, peace and justice.

There were indications, meanwhile, that when the heads of government begin to focus on economic issues rather than pressing political problems, the question of how to deal with stabilizing the dollar may cause disagreements.

Exploratory suggestions by Baker that the summit consider new ways to tighten coordination of economic policy received a frosty reception from most of his fellow finance ministers.