Cameron said Russia must join the West in seeking a democratic transition from the autocratic government of Bashar al-Assad, "so that people in Syria can have a government that represents them, rather than a government that's trying to butcher them."
Putin and Cameron discussed Syria at length Sunday in Downing Street and couldn't paper over their differences in an at-times frosty news conference afterward.
Putin was due to hold bilateral meetings Monday with President Barack Obama and French President Francois Hollande before all G-8 leaders discuss the Syrian impasse over a working dinner at the summit. Putin has sharply criticized Obama's announcement last week that Washington plans soon to ship weaponry to rebel forces, seeking to tip the military balance against the Russian-supplied Syrian army.
Cameron said Russia and the West need to unite behind a diplomatic push that transitions Assad from power in favor of a new governing coalition committed to elections. Russia is fearful of losing its only Mediterranean port in Syria as part of any government overthrow.
The British leader conceded his worries that the Syrian opposition forces included anti-democratic forces and terrorists, making any effort to arm the rebel side problematic. He said Britain had yet to identify a plan that would allow British weaponry to bolster the moderate opposition, not extremists linked to Lebanon's Hezbollah militia and Iran.
"We shouldn't accept that the only alternative to Assad is terrorism and violence. 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," Cameron said.