(CNN) -- Not long after the United States said it will start arming Syrian rebels, Syria's longtime ally Russia fired back by saying the move supports those "who kill their enemies and eat their organs."
The latest dispute sets a riveting backdrop to the Group of Eight summit in Northern Ireland on Monday, where the Syrian civil war will likely top the agenda among eight of the world's most powerful countries.
In one corner, the United States, the United Kingdom and France say rebels need more help in ousting a 42-year dynasty and ending a regime that crushes dissent with lethal force.
In the other corner, Russia says its supply of arms to Syrian regime isn't nearly as bad as sending weapons to the rebels.
"I believe you will not deny that one should hardly back those who kill their enemies and eat their organs. ... Do you want to support these people? Do you want to supply arms to these people?" Russian President Vladimir Putin asked Sunday.
He was referring to a widely circulated video that allegedly showed a rebel fighter eating the heart of a dead soldier. The video, posted by a group loyal to the Syrian government, raised questions abut the rebels' credibility, even though the Syrian opposition widely condemned the act.
Russia and Syria have an alliance dating back to the Cold War, and the Syrian government has been a longtime purchaser of Russia's weapons.
On Monday, Putin and U.S. President Barack Obama will meet one-on-one to discuss the war that has now killed more than 92,000 people -- including thousands of children.
Chaos on the ground
While world leaders struggle to form a unified front, the seesaw battles rage on inside Syria -- and at a staggering price. About 5,000 Syrians are killed every month, the United Nations said.
The opposition once controlled the Damascus suburb of Yarmouk, which served as a Palestinian refugee camp. But Palestinian fighters supporting the regime say they're taking the area back.
"We will keep fighting until we get rid of Jabhat al-Nusra and al Qaeda and all other insurgents in Syria," fighter Abu Ihad told CNN's Fred Pleitgen in Yarmouk.
The pro-government fighters said they're angry at the United States' decision to arm the opposition, especially since members of Jabhat al-Nusra -- or al Nusra Front -- have joined the rebels. The United States has designated al-Nusra Front as a terrorist group with links to al Qaeda in Iraq.
In recent weeks, the rebels have suffered a series of devastating setbacks. Their loss of the stronghold Qusayr coincided with the arrival of Hezbollah fighters supporting Syrian troops.
Syrian rebels have pleaded for anti-tank and anti-aircraft weapons, saying they are outgunned by al-Assad's military.
The U.S. has a new game plan
The Obama administration announced last week it will start arming rebels because Syria crossed a "red line" by using chemical weapons -- including sarin gas -- against the opposition.
Obama has not detailed the increased military support, but Washington officials told CNN that the plan includes providing small arms, ammunition and possibly anti-tank weapons to the rebels.
The chairman of the Senate Foreign Relations Committee told CNN's Candy Crowley there is a strong consensus on arming Syrian rebels.
"As the Foreign Relations Committee voted nearly a month ago on a strong bipartisan vote of 15-3 ... we believe the rebels need to be armed, the moderate elements of those rebels," Sen. Robert Menendez said.
"Public intelligence sources have said that we've come to know who, in fact, we could ultimately arm. And the reality is we need to tip the scales, not simply to nudge them. And the president's moving in the right direction."
Britain has not decided whether to provide weapons to rebels but has provided technical assistance and training alongside the United States, France and its other allies.
"I'm in no doubt that responsibility lies with President Assad. It is the onslaught that he is inflicted on his own people which is the primary cause of the suffering, the humanitarian catastrophe and the deaths we have seen," British Prime Minister David Cameron said Sunday.
Russia's president said he believed both sides were responsible for the bloodshed. Putin said he hoped the G8 summit this week help broker a peace deal to end the carnage.
But it's unclear how many more lives may be lost in the meantime.
CNN's Igor Krotov and Kevin Liptak contributed to this report.
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