The United States today expressed "serious concerns" over the possible consequences of yesterday's Israeli air attacks on Syrian helicopters, and urged Israeli Prime Minister Menachem Begin to shift Israel's emphasis from military action in Lebanon to political solutions.

U.S. Ambassador Samuel Lewis, in a one-hour meeting with Begin, told the Israeli leader that the Reagan administration is "terribly worried" about Israel's first direct military action in support of Christian forces in central Lebanon, according to diplomatic sources.

Lewis reportedly discussed with Begin how to "move from the military plane to the political plane," an informed source said, but dealt more in generalities than specifics. The meetings was held at Lewis' request.

For his part, Israeli sources said, Begin asked Lewis to convey to a message to U.S. Secretary of State Alexander M. Haig Jr. that Israel will not permit the Syrians to "annihilate" Christmas in the mountains northeast of Beirut.

The Israeli interpretation of the meeting was that Lewis did not convey any pressure from Washington on Israel as a result of yesterday's Israeli Air Force attacks in the Bekaa Valley.

Since Haig's visit to Jerusalem on April 6, Israeli officials have said it was their impression that they would no longer be publicy criticized by the United States for using U.S.-made weapons in Lebanon, providing their actions did not interfere with diplomatic initiatives seeking to end the Syrian-Christian fighting, and providing the actions were necessary for maintaining security along Israel's northern border.

Begin said yesterday that one purpose of the shooting down of the helicopters was to warn Syria against moving father south toward the Israeli-supported Christian enclave.