International tensions over Syria looked set to dominate the G8 summit in Northern Ireland as world leaders flew in for an annual gathering which will also focus on measures to kick-start global growth.
Prime Minister David Cameron was holding private talks today with Barack Obama - who has said he may send weapons to opposition forces seeking to oust Syria's Bashar Assad - before the US president meets his Russian counterpart Vladimir Putin, Assad's most powerful international backer.
The dramatic gulf between Russia and the West over Syria was laid bare in talks between Mr Cameron and Mr Putin at Downing Street yesterday, when the Russian president warned the international community to be wary of arming militants who "eat the organs" of their enemies.
Mr 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.
He stressed that no decision has yet been taken for the UK to supply arms to the rebels, and repeated his promise that he "would not stand in the way" of the House of Commons voting on any such issue.
London mayor Boris Johnson warned today against arming the 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 at Lough Erne, 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."
While acknowledging the "big differences" between the West and Moscow - which has been supplying arms to Assad - Mr Cameron insisted: "Where there is common ground is that we all see the need for a peace conference, a peace process and a transition to a different regime in Syria. There is some common ground there and that's what we should work on here at the G8."
Mr Obama arrived on board Air Force One at Belfast's Aldergrove International Airport this morning for his first visit to Northern Ireland, accompanied by wife Michelle and daughters Malia, 14, and Sasha, 11.
He was flown to Belfast City Airport by helicopter and then travelled to the Waterfront Hall conference centre where he met First Minister Peter Robinson and Deputy First Minister Martin McGuinness before addressing an invited audience of 2,000 mostly young people.
The President said 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'," said Mr Obama. "You are their blueprint to follow, you are their proof of what is possible. Because hope is contagious."
He promised that the US will support those who choose the path of peace in Northern Ireland "every step of the way", telling the young people in his audience: "Peace is indeed harder than war... A bullet need happen only once, but for peace to work we must be reminded of its existence again and again and again.
"And that's what we need from you, from every young person in Northern Ireland and around the world.
"You must remind us of the existence of peace, of the possibility of peace. You have to remind us of hope again and again and again."
After talks with Mr Cameron, Mr Obama was travelling later to the luxury golf resort of Lough Erne, near Enniskillen, to join EU leaders in launching negotiations on a free-trade deal between Europe and America.
Negotiations on the EU/US pact - dubbed an "economic Nato" - are expected to last up to 24 months. Mr Cameron believes a successful conclusion could be worth up to £10 billion to the UK economy, or £380 for every British household.
The annual summit of the G8 - which brings together leaders of the UK, US, France, Germany, Italy, Canada, Japan and Russia - was taking place behind tight security, with 8,000 police mounting a ring of steel around the venue.
Despite the international focus on Syria, Mr Cameron made clear that he wants the summit - the first G8 to be hosted by the UK since Gleneagles in 2005 - to deliver progress on his economic priorities of trade, tax and transparency.
When formal talks get under way later today, he is hoping to forge agreement on greater sharing of tax information to prevent companies and individuals avoiding paying their fair share by hiding cash in offshore bolt-holes.
At the weekend the tax transparency drive was boosted by the agreement of 10 British overseas territories and Crown dependencies to sign up to new OECD standards.
Mr Cameron said: "I think we have made huge progress already, even before leaders have arrived here in Northern Ireland.
"I know the scepticism there is around the country about leaders meeting, a lot of words, will things happen?
"I want to be absolutely clear that my agenda here is about helping hard-working families right here in the United Kingdom.
"We've already actually gathered billions of pounds of extra tax revenue because of this agenda, which means that I can keep taxes down on hard-working families across the United Kingdom.
"We are going to achieve more on that. If we sign trade deals and start trade negotiations here at this meeting in Northern Ireland, that will cut prices and mean a wider range of goods in shops here in the United Kingdom and jobs here in the United Kingdom.
"So my focus is very clear - getting agreements, signing deals, making progress on issues that will help hard-working families right here in the United Kingdom. For me, that's what the agenda of this G8 is all about."