US President Barack Obama has vowed to step up efforts to eliminate Islamic State and prevent more attacks like those in Paris, while urging Russia's Vladimir Putin to focus on combating the jihadist group in Syria.
A White House official said Mr Obama and Mr Putin agreed in a 35-minute meeting on the sidelines of the Group of 20 (G20) meeting in Antalya, Turkey on the need for a political transition in Syria, saying events in Paris had made it all the more urgent.
The two-day summit brings Obama and fellow world leaders just 500 kilometres from Syria, whose four and a half year conflict has transformed Islamic State into a global security threat and spawned Europe's largest migration flows since World War Two.
Mr Obama described the killing of more than 120 people in Paris, claimed by the radical Sunni militant group, as an attack on the civilised world, and said the United States would work with France to hunt down those responsible.
"The skies have been darkened by the horrific attacks that took place in Paris just a day and a half ago," Mr Obama said.
"We will redouble our efforts, working with other members of the coalition, to bring about a peaceful transition in Syria and to eliminate Daesh (the Arabic acronym for ISIL) as a force that can create so much pain and suffering for people in Paris, in Ankara, and in other parts of the globe," he said.
US-led efforts to combat Islamic State were complicated when Russia joined the conflict a month and a half ago, targeting what the West says are mainly areas where foreign-backed fighters are battling Assad, Moscow's ally, rather than Islamic State.
The United States, Turkey and their allies want Assad out.
Mr Obama huddled with Mr Putin during a working lunch and the two agreed on the need for a Syrian-led transition including UN-mediated talks, the White House official said.
Putin and Obama talked "extensively", Russian news agencies cited top Kremlin foreign policy adviser Yuri Ushakov as saying.
"Strategic objectives relating to the fight against the Islamic State are, in principle, very similar, but there are differences on the tactics side," Mr Ushakov said.
Their meeting built on progress in Vienna, where foreign ministers on the weekend outlined a plan for a political process in Syria leading to elections within two years, although differences over Assad's fate remain.
An official said the presidents discussed efforts to find a solution to the conflict, which had been made more pressing by the attacks in Paris.
The official said Mr Obama and Mr Putin agreed on the need for a Syrian-led and Syrian-owned political transition, which would be proceeded by UN-mediated negotiations between the Syrian opposition and regime, as well a ceasefire.
A rare opportunity
The Paris attacks again demonstrated how Islamic State posed a threat far beyond its strongholds in Syria and Iraq.
Washington already expected France to retaliate by taking on a larger role in the US-led coalition's bombing campaign against Islamic State.
But the United Nations Secretary-General Ban Ki-moon urged caution over how Europe reacts to the Paris attacks, saying any action should avoid fanning the flames of hatred.
Speaking at the G20 summit in Turkey, where major world leaders were discussing ways to strengthen international security, Mr Ban called on European countries to continue helping refugees flowing from Africa and the Middle East, despite reports one of the Paris attackers may have posed as an asylum seeker.
"At this time of heightened tension I caution against action that would only perpetuate the cycle of hatred and violence."
Mr Ban said he welcomed the renewed sense of urgency to find a solution to the war in Syria after the Paris attacks, adding the world had a "rare moment" of diplomatic opportunity to end the violence.
Mr Obama wants to coax other European and Middle Eastern countries into more tangible steps to show their military commitment and was due to hold a bilateral meeting with Saudi Arabia's King Salman. Last month, the two leaders affirmed the need to cooperate against Islamic State.
Mr Obama said he also discussed in a meeting with Turkish President Tayyip Erdogan coordinating efforts to fortify the border with Syria, which Islamic State has used to smuggle supplies and foreign fighters.
The coordinated attacks by gunmen and suicide bombers in Paris put Mr Obama and other leaders of the world's major economies under increased pressure to find common cause.
It remained to be seen, however, whether Washington itself had an appetite for much deeper involvement, after already stepping up air strikes and committing small numbers of special operations troops to northern Syria to advise opposition forces in the fight against Islamic State.
The Paris carnage, in which 129 people were killed in attacks on a concert hall, restaurants, bars and outside a sports stadium, also poses a major challenge for Europe, with populist leaders rushing to demand an end to an influx of refugees and migrants from the Middle East and Africa.
In a diplomatic coup for Europe and for Turkey, the G20 leaders will agree that migration is a global problem that must be addressed in a coordinated way, according to a draft communique seen by Reuters, although it has yet to be accepted by all and was due to be published on Monday (Turkish time).
Europe and Turkey, the most-heavily hit by the crisis, had been pushing for the G20 to recognise the issue as a global problem and help to deal with it financially, despite opposition from China, India and Russia. A million migrants from the Middle East and Africa are expected to come to Europe this year alone.
According to a separate statement due to be released later on, a draft of which was seen by Reuters, they also agreed to step up border controls and aviation security in the wake of the Paris attacks, which they condemned as "heinous".