Grisly Video, Putin Criticism Further Tarnishes Image of Syrian Rebels

Russian leader calls Kerry a liar; Video depicts execution of regime forces.

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Determining precisely who is fighting against the Bashar al-Assad regime has come under increased scrutiny in recent days, as President Barack Obama continues to implore lawmakers at home and world leaders abroad that military action is now necessary.

[READ: Putin Puts Pressure on U.S. For Proof of Chemical Weapons Attacks]

Russian President Vladimir Putin called U.S. Secretary of State John Kerry a liar on Wednesday. This was the same day Kerry briefed senators on classified information he says shows direct proof of the Assad regime's use of chemical weapons. Kerry also answered questions before the House Foreign Affairs Committee addressing lawmakers' concerns about increased extremism among the rebel fighters.

"This was very unpleasant and surprising for me," Putin said before his human rights council. "We talked to [the Americans] and we assume they are decent people, but he is lying and he knows he is lying."

Putin spoke the day before he begins hosting the G-20 economic summit in St. Petersburg.

It remains unclear precisely what Putin was referencing, though he had recalled watching Kerry answer questions before Congress on Wednesday about al-Qaida's potential infiltration of the rebel forces in Syria.

[PHOTOS: Syrian Rebels Push Back

Kerry responded to queries about the rise of an al-Qaida presence by saying that was "basically incorrect" and that the rebels were more defined by moderation.

The secretary estimated that 15 to 25 percent of the opposition are "bad guys," including the al-Nusra front, which the U.S. has deemed a terrorist organization.

White House Deputy National Security Adviser Ben Rhodes told reporters aboard Air Force One en route to Russia that U.S. policy has always been to isolate the moderate fighters for support.

"There's an element of the opposition that we believe is extremist and we're not going to work with in al-Nusra," he said. "But there's a broader majority that we believe we can empower in terms of strengthening a more moderate force."

"The fact that [Putin] has concerns about the opposition is all the more reason to invest in the political process that can bring the conflict to a conclusion," he added.

[OPINION: U.S. Should Be Wary of Arming Syrian Rebels]

The image of the rebel fighters was further tarnished by a New York Times report released Thursday, including footage of what appears to be rebel fighters led by Abdul Samad Issa executing seven Syrian soldiers. The prisoners had been stripped of their clothes and forced to crouch on the ground in front of men bearing automatic rifles. Issa, known as "The Uncle" as two of his nephews are his deputies, recites a poem before instructing his men to open fire.

The bodies were then buried in an unmarked mass grave, according to the video obtained by the Times in April. A rebel fighter smuggled the video out of Syria after he grew disgusted with their actions there, the Times reports.

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