Well before the start of Monday’s G8 summit meeting, divisions were on display over Syria. Mr. Cameron conceded that he found some elements of the Syrian opposition worrying but sought to keep open the option of arming those who want a democratic future.

Speaking on Monday in Northern Ireland, Mr. Cameron, who faces internal opposition within his coalition government to arming the rebels, said he had made no decision on the issue.

“I am as worried as anyone else about elements of the Syrian opposition who are extremists, who support terrorism, who are a great danger to our world” Mr. Cameron said. “The question is what do we do about that?”

“My argument is that we shouldn’t accept that the only alternative to Assad is terrorism and violence,'’ Mr. Cameron said. “We should be on the side of Syrians who want a democratic and peaceful future for their country and one without the man who is currently using chemical weapons against them.”

After a meeting in London on Sunday with the prime minister, Mr. Putin responded in combative style after being asked if he had blood on his hands for providing military support to the Assad government.

“One hardly should back those who kill their enemies and, you know, eat their organs,” he said, referring to a widely publicized video in which a member of an anti-Assad militia appears to bite an internal organ from a dead government soldier.

“Do we want to support these people?” Mr. Putin asked. “Do we want to supply arms to these people?”

The United States has said that it will supply some rebels with direct military aid, and Britain and France succeeded in getting the European Union to allow its ban on supplying arms to the country to expire, despite the reservations of many countries within the 27-member bloc.

On Monday, Britain’s foreign secretary, William Hague, told the BBC that there was no “palatable option” for dealing with the crisis in Syria and that “extremists” were supporting both Mr. Assad’s government and the rebel forces. The help would go to “moderates,” he said.