The two leaders met Friday in the Black Sea resort city of Sochi, with Cameron telling reporters Britain, Russia and the United States "urgently have to do more" because "the history of Syria is being written in the blood of her people."
The British prime minister also strongly endorsed an agreement earlier this week between the United States and Russia to press for peace talks between the Syrian government and opposition groups at an international conference.
Putin said he and Cameron discussed "a number of possible joint steps" to end the conflict.
At least 70,000 people have been killed in the ongoing fighting between the Syrian government and rebels fighting to oust President Bashar al-Assad, a longtime Russian ally.
Cameron said despite differences with Moscow, both countries agree on the need to "help shape transitional government that all Syrians can trust to protect them."
Also Friday, the United Nations High Commissioner for Human Rights called for urgent action.
Navi Pillay said images from a suspected massacre in the western Syrian village of Bayda are appalling and "seem to indicate a campaign targeting specific communities perceived to be supportive of the opposition."
Pillay also expressed concern about what appears to be a major military buildup near the western town of Qusayr, warning there could be more atrocities if the area is overrun.
Syrian opposition groups last week accused Syrian forces loyal to Mr. Assad of killing at least 150 civilians, including women and children, in the village of Bayda. Bayda is mainly inhabited by Sunni Muslims who dominate the country's rebel movement.
The U.S. State Department said Washington was "appalled by horrific reports" of the many civilians killed in the attack and condemned the "atrocities."
Meanwhile, Turkish Prime Minister Recep Tayyip Erdogan told U.S. broadcaster NBC News "it is clear" Syrian President Assad has used chemical weapons against opposition forces. Mr. Erdogan said, the regime has "used about 200 missiles according to our intelligence and it can be even more but not less.''
Clashes continued to break out across Syria. The Local Coordination Committees, an opposition activist group, said at least 22 people were killed in fighting early Friday in the capital of Damascus and the central city of Homs, as well as Aleppo, Hama, and Daraa.
Also Friday, amateur video posted on the Internet by opposition activists claimed to show dozens of people wading through a river to escape fighting in the central Syrian town of Halfayeh. A voice, off camera, said the people were fleeing to ``escape a massacre'' by government forces.