Charles Dharapak/AP Photo

America's top diplomat at the United Nations, Ambassador Samantha Power, told an audience today that diplomacy alone is no longer enough to stop Syrian President Bashar al-Assad from using chemical weapons.

"We have exhausted the alternatives," Power said in a 20-minute speech at the Center for American Progress.

For more than a year, she said, the United States has tried to use diplomacy to warn the Assad regime of the consequences of using chemical weapons.

Despite such efforts, Power said, "Assad began using chemical weapons on a small scale several times this year" - a move that caused the U.S. to "redouble" its diplomatic efforts.

The U.S. even shared evidence that it believed could convince Russia or Iran, but to no avail, she added.

"Russia, often backed by China, has blocked every single action," Power said.

She argued that the time for diplomacy alone ended Aug. 21, when, she said, Assad staged "the largest chemical weapons attack in quarter century."

She noted that Syrian forces even launched the attack while U.N. inspectors were on the ground.

Without holding Assad accountable for his use of chemical weapons, Power said, getting both sides back to the table for a political solution will be impossible.

"At this stage, the diplomatic process has stalled because one side has been gassed and the other side feels it has gotten away with it," she said.

Power seemed skeptical about the possibility of responding with new sanctions and had harsh words for lack of stronger action by the U.N. Security Council.

"We could try again to pursue economic sanctions, but - even if Russia budged - would more asset freezes, travel bans and banking restrictions convince Assad not to use chemical weapons again when he has a pipeline to the resources of Hezbollah and Iran?" she asked. "Does anybody really believe that deploying the same approaches we have tried for the last year will suddenly be effective?"

Power said the U.S. would have preferred to find a solution working through the United Nations.

"We would if we could, but we can't," she said.

She highlighted Russia's role in blocking action. Beyond three Security Council resolutions Russia has vetoed over the last two years, in the last year Russia has blocked at least three statements expressing humanitarian concern and calling for humanitarian action to besieged cities in Syria. In past two months, Russia has blocked tw0 resolutions condemning the generic use of chemical weapons and two prospective press statements expressing concerns about their use, Power said.

Calling a belief that Russia will change its position as "naïve" Power said that working through the Security Council simply is no longer an option.

"The Security Council we need is not the Security Council we have," she said.