Cameron is seeking to forge an international consensus on handling the unrest as he hosts the leaders of the world's top industrialised nations in Lough Erne, Northern Ireland, from tomorrow.
Washington has upped the ante on Syria by vowing to send military aid to rebel forces battling to topple President Bashar al-Assad after saying it had proof that his regime had crossed a "red line" by using chemical weapons on a small scale.
American officials will not reveal exactly what military support will go to the rebel Supreme Military Council, although by many estimates it will initially be assault rifles and ammunition.
Russian Foreign Minister Sergei Lavrov backed Assad today, saying there was no need for the Syrian leader to use chemical weapons against the rebels because his forces were making steady advances on the ground anyway.
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"What sense is there for the regime to use chemical arms -- especially in such small amounts?" he asked.
Cameron had to meet Putin at the back entrance to Downing Street due to a large protest against the Turkish government blocking the main gates.
The president's plane arrived late at a military airbase in west London and was further held up by the protest.
Putin and Cameron shook hands and posed for photographs before heading inside.
Ahead of the talks, Cameron said it was essential to assist the moderate Syrian rebels prepared to work with the West before Al-Qaeda-linked extremists gained the upper hand in the opposition.
"But there are elements of the Syrian opposition who want to see a free, democratic, pluralistic Syria that respects the rights of minorities, including Christians, and we should be working with them."
Cameron has not said whether he favours sending weapons to the rebels, but he believes the lifting in May of a European Union arms embargo was essential to put pressure on Assad.