Barack Obama has conceded there are "differing perspectives" between Washington and Moscow on Syria, following talks with Vladimir Putin.
The gulf between Russia and the West was underlined as leaders of the world's most powerful countries gathered in Lough Erne, Northern Ireland, for a G8 summit which was overshadowed by international tensions over the civil war in Syria.
David Cameron spoke privately with Mr Obama on Monday - who has said he may arm the rebels seeking to oust the regime of Bashar Assad - ahead of the US president's talks with Putin, Assad's most powerful international backer.
On Sunday the Russian president warned the international community to be wary of arming militants who "eat the organs" of their enemies.
But Obama tried to find some common ground, saying after the talks: "We do have differing perspectives on the problem but we share an interest in reducing the violence and securing chemical weapons and ensuring they are neither used nor are they subject to proliferation.
"We want to try to resolve the issue through political means if possible."
Meanwhile, Assad used an interview with German newspaper Frankfurter Allgemeine Runschau to warn Europe would pay a price if it decided to arm Syrian rebels.
The Syrian leader also disputed the US assertion, backed by the UK, that his forces had used chemical weapons.
He told the newspaper: "If the Europeans ship weapons, Europe's backyard becomes a terrorists' place, and Europe will pay a price for it."
Speaking in Arabic translated to German, he warned: "Terrorists will return to Europe with fighting experience and extremist ideologies."
The US has claimed chemical weapons, including the sarin nerve agent, had been used by government forces.
But Assad said: "Weapons of mass destruction are capable of killing hundreds, thousands at once. That's why they are used.
"That's why it is not logical to use chemical weapons to kill a number of people that can be achieved through conventional weapons."
He added: "If Paris, London and Washington had only one piece of evidence backing up their allegations, they would have unveiled it to the world."
David Cameron said this morning he was "as worried as anyone" about terrorist and extremist elements in the opposition forces, but insisted it was right to engage with the Syrian crisis and seek to put pressure on both sides to attend peace talks to discuss a transitional government for the country.
And speaking at Belfast's Waterfront Hall in Monday morning, Mr Obama said that 15 years after the Good Friday Agreement Northern Ireland was setting an example for people all over the world seeking to escape apparently intractable religious, national and ethnic conflicts.
"They are wondering 'Perhaps, if Northern Ireland can achieve this, we can too'," he said.
"You are their blueprint to follow, you are their proof of what is possible. Because hope is contagious."
It was the conflict in Syria which dominated Mr Obama's talks shortly afterwards with Mr Cameron as the two leaders travelled in the President's armoured limousine, nicknamed The Beast.
And the two-year civil war was set to be the main topic of discussions between leaders of the G8 countries - the UK, US, France, Germany, Italy, Canada, Japan and Russia - at the official summit dinner in the luxury Lough Erne golfing resort on Monday evening.
In London, mayor Boris Johnson warned against arming the anti-Assad rebels, saying that British weapons could end up in the hands of "al Qaida-affiliated thugs". But Foreign Secretary William Hague said Britain must protect the Syrian opposition from being "exterminated", warning that the crisis was "on a trajectory to get worse".
In a round of TV interviews, Mr Cameron said: "Let's be clear - I am as worried as anybody else about elements of the Syrian opposition, who are extremists, who support terrorism and who are a great danger to our world.
"The question is what do we do about it? My argument is that we shouldn't accept that the only alternative to Assad is terrorism and violence.
"We should be on the side of Syrians who want a democratic and peaceful future for their country and one without the man who is currently using chemical weapons against them.
"What we can try and do here at the G8 is have further pressure for the peace conference and the transition that is needed to bring this conflict to an end."