WASHINGTON — With evidence that powerful new weapons are flowing to both the Syrian government and opposition fighters, the bloody uprising in Syria has thrust the Obama administration into an increasingly difficult position as the conflict shows signs of mutating into a full-fledged civil war.

Secretary of State Hillary Rodham Clinton said on Tuesday that the United States believed that Russia was shipping attack helicopters to Syria that President Bashar al-Assad could use to escalate his government’s deadly crackdown on civilians and the militias battling his rule. Her comments reflected rising frustration with Russia, which has continued to supply weapons to its major Middle Eastern ally despite an international outcry over the government’s brutal crackdown.

“We have confronted the Russians about stopping their continued arms shipments to Syria,” Mrs. Clinton said at an appearance with President Shimon Peres of Israel. “They have, from time to time, said that we shouldn’t worry; everything they’re shipping is unrelated to their actions internally. That’s patently untrue.”