ABC News' Adam Teicholz reports:

In a wide-ranging, exclusive interview with George Stephanopoulos this morning on "This Week," U.S. Ambassador to the United Nations Samantha Power had harsh words for Russian President Vladimir Putin in the wake of pro-Russian soldiers' recent takeovers of government buildings in several eastern Ukrainian cities.

Asked by Stephanopoulos if the U.S. believes that Putin "wants" Eastern Ukraine, Power responded, "I think the actions that he is undertaking certainly give credence to that idea. But I will say in the conversations that we have, of course, they keep insisting, 'No, that's not what we want, that's not what we want.' But everything they're doing suggests the opposite."

"The leadership in Ukraine have made very clear that they're prepared to have a conversation about autonomy and decentralization," Power added regarding eastern Ukraine, but Putin's continued provocations "make you think that a military solution is what" he is seeking.

By contrast, Power issued a relatively tempered response to Saturday's revelations of potential new chemical weapons attacks in Syria, saying the administration is still trying to confirm the reports.

"We are trying to run this down. So far, it's unsubstantiated," Power said. "But we've shown, I think in the past, that we will do everything in our power to establish what has happened and then consider possible steps in response."

Asked whether the U.S. had any option but to strike militarily following President Obama's "red line" comments and Syria's promise to turn over its chemical arsenal last fall, Power responded that President Obama finds the use of chemical weapons to be "alarming" and "outrageous," and the U.S. would have to "look at our policy on this."

"That's why he put the credible threat of military force on the table, that's why we've been able to destroy and remove more than half of Syria's chemical weapons up to this point," Power said. "But certainly the point of what we've done so far is to prevent further use. We weren't just removing for removing sake, it was to avoid use."

Power was a prominent proponent of aggressive action to protect civilians in Syria against attacks by Assad forces in 2012 and 2013, but has been quieter on the subject since a Russian-brokered deal was put in place last September that allowed the Syrian regime to peacefully dismantle its chemical stockpile.

Remembering Rwanda

Power recently returned from Africa, where she visited Rwanda on the 20th anniversary of that country's mass slaughter of the Tutsi ethnic minority. She also stopped in the Central African Republic, or CAR, where experts fear the outbreak of Rwanda-style genocide against Muslims by Christians today.

She said the crisis in CAR involves American national security interests that go beyond the mere defense of our values. The "Muslim population now in the Central African Republic has been displaced," Power said, "and we know how dangerous that can be as unsavory elements get in [and] try to exploit" the persecution and disenfranchisement of religious minorities. Muslims make up about 15% of the Central African Republic's populace. "This is a population that can be radicalized," Power cautioned.

Power is the author of 2002's "A Problem from Hell," a Pulitzer-prize winning book about American inaction in the face of genocide. She told "This Week" that in the 20 years since the horror in Rwanda, the world has learned that "we can't affect people's desire to want to kill one another on ethnic, religious or other grounds." But, she offered hopefully, "we're much quicker" to respond to try to stop it.

In "A Problem from Hell,"the former journalist and professor Power admonished, "The United States should not frame its policy options in terms of doing nothing or unilaterally sending in the Marines." That stark choice, she said, meant in the real world of political decision-making that the answer too often would be "doing nothing" in the face of human suffering abroad.

Now, Power is in a position to frame these policy choices herself, and she expressed optimism that in the United States, at least, her message has gotten through.

"We've learned the lesson that you can't make the choice one, between doing nothing, on the one hand, and sending the U.S. Marines, on the other. There's lots in between," Power said.

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