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As Barack Obama addressed the G20 summit in Turkey, he reiterated the United State's solidarity with "our French allies", in the wake of the "heinous attacks" that took place in Paris on Friday.
The president said they were a setback back in the fight against Islamic State, but the coalition led by the US was making progress in bringing down the militant group in Syria and Iraq.
"We have always understood that this would be a long-term campaign. There will be setbacks and there will be successes."
"Even as we grieve with our French friends, however, we can't lose sight that there has been progress being made."
Mr Obama said despite some calls some US ground troops in Syria, it is his view that would be a mistake.
"We can retake territory and, as long as we keep our troops there we can hold it, but that does not solve the underlying problem of eliminating the dynamics that are producing these kinds of violent, extremist groups."
The rise in global acts of terrorism undermines international peace and security, G20 officials have said.
A statement released on behalf of world leaders at the G20 summit also warned that incidents like Friday's attacks in Paris endangered efforts to strengthen the economy around the world.
The G20 group remains committed to cutting off the sources of funding for terrorist groups, the statement added, and said terrorism should not be associated with any religion, nationality or ethnic group.
David Cameron has said the "horrific" attacks in Paris and others "underline the threat we all face, a threat to our values, to our way of life and a threat we must defeat together".
Speaking after meeting with Russia's President Vladimir Putin at the G20 summit in Turkey, he said "a very big gap" remains between the West and Russia over tackling the crisis in Syria, but there are signs of a willingness to compromise on all sides.
On the issue of Russia's airstrikes over Syria, which the Kremlin has said are targeting Islamic State, Mr Cameron said there are some signs they are focusing more on the group and it is hoped this would continue.
However, he also acknowledged that "a few extra bombs and missiles won't transform the situation".
He also stressed the need for a government with the confidence of all Syria's ethnic and religious communities to bring about peace.
Mr Cameron also announced that he will co-host a donors' conference in London next year to raise "significant new funding" to address the refugee crisis Syria and neighbouring countries.
The G20 summit has agreed "important steps" to cut off terrorist financing and counter extremist ideologies, David Cameron has said.
The Prime Minister was speaking at a press conference after discussing counter-terrorism measures with Russian President Vladimir Putin.
He said he had urged Putin to work with the international community to support the transition in Syria away from President Bashar al-Assad, and had made it clear that he believes bombing moderate opposition groups in the country was a "mistake".
Everyone recognises the need for compromise over Assad's future, he added, and said he feels the "gap" on views about what that should be has now "reduced".
He said there are still "very big disagreements" with Putin over both Syria and Ukraine - but said the two countries should work together where there is agreement.
Russia and Britain should join forces in the fight against terrorism, Russian President Vladimir Putin told David Cameron.
Speaking to the British Prime Minister on the sidelines of the G20 summit in Turkey, Putin urged cooperation, saying Russia appreciates Britain's past work in fighting terrorism.
Acknowledging that relations between the nations were "not of the best", he told Cameron that Friday's terror attacks in Paris demonstrated the need for them to "join efforts in preventing terrorism".
"Russia has analysed the positive groundwork that you have done in the past and to look into the future as to the way we should develop our relations," he said.
Cameron passed on his condolences for the 224 people killed when a Russian passenger jet came down over Egypt's Sinai province.
World leaders met in Turkey today for the G20 summit as France began sustained airstrikes on the so-called Islamic State's de facto capital city of Raqqa in Syria.
ITV News International Affairs Editor Rageh Omaar reports on the world's Security Strategy.
There was one key priority on the agenda as world leaders gathered at the G20 summit in Turkey - how to tackle so-called Islamic State.
The G20 summit is the first meeting of world leaders since the deadly attacks on Paris.
Although many of the leaders had differences over how to proceed in Syria against the militant group, they all seemed to agree that IS poses a significant danger and that action needs to be taken quickly.
ITV News' International Affairs Editor Rageh Omaar reports:
US President Barack Obama and Russia's President Vladimir Putin talked to each other for more than 30 minutes in an informal meeting at the G20 summit in Turkey.
Svetlana Lukash, the Kremlin Sherpa to the G20, tweeted that the two had been talking during a break of the summit that is being held in Antalya.
She did not disclose any details about what the two discussed, but did post a photo of Obama and Putin sitting near to each other.
World leaders fell silent in tribute to the victims of the terror attacks in Ankara and Paris at the G20 summit today.
A total of 102 people were killed in suicide bomb blasts in the capital of Turkey last month, while the death toll from Friday's coordinated attacks across France's capital city stands at 129.
The G20 is a forum consisting of governments and central bank governors from 20 major economies around the world to meet and discuss issues affecting the world.
The rise of militant extremism is high on this year's agenda.
US president Barack Obama has arrived in Turkey for the G20 summit.
On Sunday evening, he will hold a bilateral meeting with Saudi Arabia's King Salman, the White House has said.
The two countries are partners in a coalition bombing Islamic State militants in Syria.