US Defense Secretary Caspar W. Weinberger said yesterday that the US government holds Syria responsible for the recent attack that killed 239 American servicemen in Lebanon.
In a press conference statement that appeared to go beyond earlier United States pronouncements, Mr. Weinberger said those behind the Oct. 23 suicide truck-bomb mission were ''basically Iranians with the sponsorship and knowledge and authority of the Syrian government.''
Weinberger refused to characterize this as an act of war, but did not rule out retaliation by US forces. Nor did he acknowledge that recent retaliatory air strikes by Israeli and French forces following similar bomb attacks on their positions in Lebanon constituted acceptable retribution by the US, even though these were directed against the same pro-Iranian, Syrian-backed group by close allies of this country.
While other officials in recent days have been playing down the notion of a US military response to the attack on the marine headquarters, the defense secretary thus left open the possibility to such an event.
The defense secretary began the half-hour exchange with Pentagon reporters by analyzing last week's congressional wrap-up of the 1984 defense budget, and in general described himself as quite pleased.
''We have endorsement, approval, and appropriations for all of the President's major programs and weapons systems except the chemical warfare weapons that were requested,'' Weinberger said. Congress trimmed administration requests involving some 380 items by $11.1 billion, or just over 4 percent of the $261 billion sought by the Pentagon.
The primary effect of those cuts, he said, will be to slow the rate of new weapons purchases. This will raise the cost of those weapons, he said, because the Pentagon's budget figures were based on buying in the most ecomonical quantities.
For example, the administration had asked for 14 weapons procurement programs to be designated as ''multiyear,''and argued that this would save money in the long run. Weinberger said that by denying eight of these multiyear requests in favor of single-year procurement, the overall cost (assuming Congress continues to fund these weapons, which is usually the case once procurement starts) will be $1.3 billion higher.
On other matters, the defense secretary offered these comments:
* He said there is ''no substance'' to recent press reports that the US might delay the deployment of some intermediate-range nuclear missiles in Europe. ''There's been no change to the deployment schedule,'' he said. The Air Force reported yesterday that a Tomahawk cruise missile (the type to be deployed in Europe) had crashed in a recent test flight, but Weinberger said there was ''no technical reason'' to delay deployment.
He added that although ''the Soviets may leave in dramatic fashion for a short period,'' he does not think there will be a breakdown in the arms control talks at Geneva.
* Weinberger would not comment on the state of Soviet leadership, which some US officials have recently expressed concern about. But he did say it was ''extremely significant'' that Soviet leader Yuri Andropov was not present at recent official functions and ceremonies in Moscow. And he acknowledged that ''certainly relations (between the US and the Soviet Union) are not good.''
* The defense secretary said he had seen ''bits and pieces'' of the recent television program on nuclear war titled ''The Day After.'' He described the conditions portrayed as ''horrible,'' and said, ''It is exactly why we are doing the kinds of things that we think are essential to prevent the horrors depicted in that film.''
In particular, Weinberger emphasized the need for a ballistic missile defense program as outlined last spring in President Reagan's controversial ''star wars'' speech. ''To my mind, this offers the greatest hope of all that we can render impotent these weapons,'' Weinberger said.