National Security Adviser Susan Rice on Monday said that Syria's use of chemical weapons, if left unpunished, would be a threat to U.S. security.

"Assad’s escalating use of chemical weapons threatens the national security of the United States," said Rice in an address at the New America Foundation.

"Every time chemical weapons are moved, unloaded, and used on the battlefield, it raises the likelihood that these weapons will fall into the hands of terrorists active in Syria, including Assad’s ally Hezbollah and al Qaeda affiliates," she added. "That prospect puts Americans at risk of chemical attacks targeted at our soldiers and diplomats in the region and even potentially our citizens at home."

Her comments came as Russia floated a proposal to allow Syria to allow international monitors to secure their chemical weapons in hopes of averting a U.S. military strike.

The Obama administration is pressing Congress to authorize a military attack after Syrian President Bashar Assad used chemical weapons on civilians last month in that country's brutal civil war.

The White House said Monday they would take a "hard look" at the Russian proposal, but that they would continue to press lawmakers to approve a strike.

Rice said the U.S. was resorting to force only as a last resort, arguing that United Nations action is "just not going to happen now."

"I was there for all of those UN debates and negotiations on Syria. I lived it. And it was shameful," she said of efforts by Russia and China to block resolutions punishing Damascus.

Rice also dismissed Assad's claims that his regime was innocent of the Aug. 21 chemical attack.

"Only the Syrian regime has the capacity to deliver chemical weapons on a scale to cause the devastation we saw in Damascus. The opposition does not," said Rice.