"Doubling down on a regime that gases its people, that barrel bombs its people, that tortures people who it arrests simply for protesting and for claiming their rights -- that's just not going to work," Samantha Power told CNN's Christiane Amanpour.

U.S. President Barack Obama said Friday that al-Assad "is worried enough that he is inviting Russian advisers in and Russian equipment," and Reuters reported Monday that Russia had begun deploying several tanks to a Syrian airfield.

"Even if you were Machiavelli and all you cared about was ISIL," Power said, using the U.S. government's preferred abbreviation for ISIS, "to support a regime like this and to not take account of the views of the vast majority of the Syrian people that want to go in a different direction is not going to either bring peace or actually succeed in defeating terrorism, which is what President Putin says his priority is."

The tens of thousands of refugees streaming across Europe's borders has put renewed focus on the war in Syria. At least 200,000 people have been killed in the more than four-year-long civil war -- activists put the number at more than 310,000.

The United States has come under increasing criticism for President Obama's decision not to significantly intervene to stop the bloodshed.

But even the President himself, Power said, "is a critic."

"You can't look at 12 million people being displaced from their homes, and desperate families washing up on shores and be satisfied with where we are. I think the challenge is to find what is the policy tool that's going to make things better. And that has proven very, very challenging."

Power, a former journalist who wrote a bestselling book on genocide, has been singled out for her role in the American policy, most recently by Washington Post Editorial Page Editor Fred Hiatt.

"The fact that the woman who wrote the book on genocide, Samantha Power ... could apparently in good conscience stay on as U.N. ambassador," Hiatt wrote, "lent further moral credibility to U.S. abdication."

With some exasperation, Power emphasized that in pursuing a political solution to the conflict, there is only so much the United States can do alone.

"Russia and Iran may be the place really where one should lodge much of that criticism for supporting a regime that is carrying out these monstrous attacks against civilians, and again fueling -- whether wittingly or unwittingly -- the rise of terrorism."

"I, like the President, am just focused on what is going to make things better in the here and now -- and it's not obvious, again, until there's a fundamental shift in the calculus by President Putin and by the Iranian regime to see that this approach of thinking you can just wipe out whole communities and think that that is going to help you deal with a terrorism problem -- that approach is just not going to be productive."

"And that is the shift that needs to occur."

(CNN)