British Prime Minister David Cameron says Russia, like all G-8 governments, has a responsibility to push opposed factions in Syria's civil war to the negotiating table as rapidly as possible and not to back a government that "butchers" its citizens.

Cameron conceded there is a chasm in opinion Monday between Russia's Vladimir Putin and leaders of some other countries about the way ahead in Syria. Putin is holding bilateral meetings Monday with U.S. President Barack Obama, French President Francois Hollande and Cameron before all discuss the Syrian impasse over a working dinner at the summit.

Cameron said Russia and the West need to unite behind a diplomatic push that transitions Syrian President Bashar Assad from power. Russia supplies Assad's army with weapons and has its only Mediterranean port in Syria.