Leaders of the world's biggest economies have vowed to co-operate in the fight against terror, after a two-day summit overshadowed by the Islamic State terror attacks in Paris.
Prime Minister David Cameron welcomed signs of a willingness to compromise over a resolution to the four-year civil war in Syria, though he acknowledged that "a very big gap" remains between Russia and the West over the fate of dictator Bashar Assad.
In a face-to-face meeting with Vladimir Putin at the G20 summit in Turkey, Mr Cameron urged the Russian President to target airstrikes in Syria at IS, rather than the moderate opponents of the Assad regime.
Mr Cameron revealed that UK intelligence and security agencies had detected and disrupted seven terror plots over the past year - including one which had not previously been made public and is understood to have been thwarted within the last month.
The plots were on a "smaller scale" than the multiple attacks which claimed 129 lives in the French capital, he said. But he warned in an interview on Radio 4's Today programme that a similar assault "could happen here".
US President Barack Obama said he was not aware of any "specific intelligence" which could have prevented the Paris outrage.
And he said it would be "a mistake" for the US to respond by sending ground troops to "take out" IS in its Syrian strongholds.
While the murders in France were a "terrible and sickening setback", Mr Obama insisted that his strategy of airstrikes combined with support for moderate groups "ultimately is going to work", though "it is going to take time".
Mr Cameron appeared to damp down expectations of a parliamentary vote on extending RAF airstrikes from Iraq into Syria.
While there was "a very strong case" for targeting IS in Syria, he recognised "that I need to do more to build this argument, build this case, to take Parliament with me".
His comments came as Labour leader Jeremy Corbyn indicated that he would not allow his MPs a free vote on military action in Syria.
Mr Corbyn said the focus must be on a political settlement for the Middle Eastern state, and said airstrikes would "probably not" help.
"We have to be careful. One war doesn't necessarily bring about peace, it often can bring yet more conflicts, more mayhem and more loss," he told ITV1's Lorraine.
Mr Cameron accepted that it was "perfectly right to say that a few extra bombs and missiles won't transform the situation".
But he said his critics were wrong to suggest that he did not have a plan for the transition of Syria to an inclusive post-Assad government which could gain the trust of all the country's ethnic and religious communities.
"Those plans are there," he said. "They need to be set out more clearly and more explanation given."
Mr Cameron said the G20 had agreed "important steps" to cut terrorist financing, counter extremist ideology, improve airport security and stop foreign fighters from travelling.
A communique agreed by the leaders condemned the IS atrocities as "an unacceptable affront to all humanity" and resolved "to prevent and suppress terrorist acts through increased international solidarity and co-operation".
Following his hour-long meeting with Mr Putin, Mr Cameron voiced hope that last weekend's peace talks in Vienna could close the gap between Western demands for Assad to go and Russia's refusal to abandon him.
"There is still a very big gap, but I think there's some hope that this process could move faster in the future than it has in the past," said the Prime Minister.
Mr Putin also met President Obama, German Chancellor Angela Merkel and Italian PM Matteo Renzi, as Western allies made a concerted bid to persuade him to accept a compromise.
The Russian president told Mr Cameron: "The latest tragic events in Paris show that we have to unite our efforts in fighting this evil, something we should have done a long time ago."
Mr Putin thanked the Prime Minister for sharing UK intelligence about the downing of a Russian Airbus in Egypt last month, which Britain believes was caused by an IS bomb. Mr Cameron insisted he was "right" to pass on information to the Russian president in a phone call soon after the disaster, which killed 224.
While Britain had "big disagreements" with Moscow over Ukraine and Syria, "where we do agree with each other we should work together", he said.