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.

On Monday, Russian President Vladimir 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.