"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 has 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 would help broker a peace deal to end the carnage.
But it's unclear how many more lives may be lost in the meantime.