The United States, following an unpublicized diplomatic appeal, publicly called on the Soviet Union yesterday to defuse an increasingly serious military confrontation between Israel and Syria in Lebanon.

Secretary of State Alexander M. Haig Jr. called the Lebanese situation "very worrisome," and other officials expressed strong concern that an action-reaction cycle of recent days may bring even more dangerous escalation.

In Jerusalem, U.S. Ambassador Samuel Lewis met for an hour yesterday with Israeli Prime Minister Menachem Begin to express "serious concern" about the potential consequences of Tuesday's Israeli air attacks on Syrian helicopters. According to Washington Post correspondent William Claiborne, Lewis discussed with Begin how to "move from the military plane to the political plane" in Lebanon.

The State Department, in a statement made public by spokesman Dean Fischer, said, "The United States has not given a 'green light' to Israel to undertake any military actions in Lebanon."

A new surge of concern in Washington was prompted by intelligence reports from the area that Syria has moved Soviet-made surface-to-air missiles into eastern Lebanon, in response to Israeli air attacks Tuesday against Syrian helicopters.

If Israel now attacks the SAM missile sites, as many expect, Syria could respond with further ground or air action.

A State Department official discounted a report broadcast on clandestine Palestinian radio that the Soviet Union, through its ambassador in Beirut, had threatened to invoke the recent Soviet-Syrian friendship treaty because of developments in Lebanon's Bekaa Valley.

On the other hand, U.S. officials said publicly and privately that the Russians have a key role, through the Syrians, in the development of the Lebanon problem.

The statement made public by Fischer called on "all governments in the world -- including the Soviets -- to do their best" to calm the situation. The spokesman refused to explain why the Soviets had been singled out, but he revealed a U.S. contact on the issue Monday in a previously unreported meeting between Undersecretary of State Walter J. Stoessel Jr. and Soviet Ambassador Anatoliy F. Dobrynin.

The United States, according to Fischer, has undertaken "intensive and sustained efforts with all governments and vitually all parties" to bring about a stable situation in Lebanon. The reason for the qualification that "virtually" all parties have been contacted is the longstanding ban on U.S. official contacts with the Palestine Liberation Organization.

The growing concern coincided with continued quiet diplomatic efforts to extend the authority of the Lebanese government over contending forces and ultimately bring about a withdrawal of the Syrian military forces.

This has long been an aim of the United States and several other Western governments, but has always proved to be beyond the reach of diplomacy. Diplomatic sources reported that there are grounds for hope in the Syrian attitude, including recent discussions in Beirut with Syrian Foreign Minister Abdul Halim Khaddam.