Syria has turned down a British request that it waive diplomatic immunity to allow its diplomats to be questioned in connection with last month's attempted bombing of an Israeli airliner at London's Heathrow airport, sources here said.

But in public statements today, Damascus moved to quash speculation that it was involved in that incident or a recent terrorist bombing in West Berlin and offered to cooperate with police investigations in both cities.

Calling such reports "an orchestrated smear campaign intended to justify Israeli and American military action against us," a statement issued by the Syrian Embassy here said that Syria "categorically denies any knowledge of, or involvement in alleged terrorist activities on British soil, and in particular the incident at Heathrow airport on April 17th, 1986."

"The embassy has so far refrained from responding to stories in the media on this subject," it said, "because it does not wish to interfere with the investigations by the British security authorities into this matter.

"We are glad to state, however, that we offered our cooperation . . . firstly because we have nothing to hide, and secondly because we are anxious to maintain the friendly relations with Britain," the statement said.

A British Foreign Office official said, "we are still considering" today's statement, and that the government here would make no comment on it.

The statement came as sources said Damascus had refused a British request that diplomatic immunity be waived to allow several Syrian diplomats to be questioned about the Heathrow incident. They said the refusal was given by Syrian Ambassador Loutof Haydar in a meeting with Foreign Office officials on Monday.

The sources said that Britain had asked for the interviews as part of the investigation of Nezar Hindawi, a Jordanian national who has been charged with the unsucessful attempt to place a bomb aboard the Tel Aviv-bound Israeli plane.

Police have said that Hindawi entered Britain illegally with a false Syrian passport.

According to sources here, the British had hoped to "negotiate" with Syria some sort of deal under which information provided by diplomats about Hindawi would not be used to expel them. Rather, they could be withdrawn quietly by Damascus.

Hindawi has been identified as the brother of Ahmed Nawaf Mansour Hasi, who two weeks ago was arrested in West Berlin for complicity in the April 5 bombing of a discotheque in which two persons were killed and 230 injured. That bombing led to the U.S. air attack against Libya for its alleged support of international terrorism.

Hasi and Farouk Salameh, a Jordanian also arrested in West Berlin, have denied involvement in the disco bombing. But they have told West Berlin police that they, and Hasi's brother Hindawi, together planned and carried out an earlier bombing, on March 29, of the German-Arab Friendship Society in West Berlin. The explosives for the March 29 bombing, they said, came from the Syrian Embassy in East Berlin.

A West Berlin court Thursday released a third Arab suspected of involvement in the bombing of the friendship society but ordered him not to leave the city pending further investigation, The Associated Press reported.

[Yesterday, Syria denied any involvement in either of the West Berlin bombings. Today, Syria's ambassador to East Germany, Fayssal Sammak, said at a news conference that Syria has asked West Germany to allow it to take part in the two investigations, Reuter reported from East Berlin.]