What Mrs. Clinton did not say, however, was whether the aircraft were new shipments or, as administration officials say is more likely, helicopters that Syria had sent to Russia a few months ago for routine repairs and refurbishing, and which were now about to be returned.

“She put a little spin on it to put the Russians in a difficult position,” said one senior Defense Department official.

Mrs. Clinton’s claim about the helicopters, administration officials said, is part of a calculated effort to raise the pressure on Russia to abandon President Bashar al-Assad, its main ally in the Middle East. Russia has so far stuck by Mr. Assad’s government, worried that if he were ousted, Moscow would lose its influence in the region…

“We are not providing Syria or any other place with things which can be used in struggle with peaceful demonstrators, unlike the United States, which regularly supplies such equipment to this region,” Mr. Lavrov said.