The G8 talks are underway at Lough Erne in Co. Fermanagh. US President Barack Obama arrived this morning in Belfast where he was given a rapturous reception.
This afternoon he will come face to face with Russian President Vladimir Putin as the talks focus on US plans to arm Syrian rebels.
The regime of Syrian President Bashar al Assad has Russian support.
The talks in Enniskillen will also be attended by the British Prime Minister David Cameron, German Chancellor Angela Merkel and French President Francois Hollande as well as the leaders of Italy, Japan and Canada.
Mr. Cameron greeted the leaders as they arrived
Watch LIVE coverage from the Lough Erne resort below. You can also view of our Storify of today's events by clicking here
Earlier, British Prime Minister Cameron said he is "as worried as anyone" about terrorist and extremist elements among opposition forces fighting to oust President Bashar Assad, and no decision has been made on arming the Syrian opposition.
The on-going conflict in Syria threatens to overshadow the summit's scheduled discussions on trade, tax and transparency, after Mr. Obama announced he was ready to start supplying weapons to the rebels seeking to overthrow Assad.
Speaking 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".
Mr. Cameron and Russian President Vladimir Putin met in London on Sunday to discuss the crisis in Syria, but appeared to have reached little common ground. At a joint news conference, Mr Cameron acknowledged there were "big differences" between the two leaders on who was to blame for the conflict but insisted they could be overcome.
Mr. Putin strongly defended the supply of arms by Moscow to Mr Assad's "legitimate" government, but also stressed he wanted to achieve a diplomatic solution to the crisis.
He said he believed the G8 summit is "one of the most appropriate means" to seek an end to the conflict.
When Mr. Putin was asked by British journalists about comments by Mr Cameron last year – that those supporting President Assad had the blood of Syrian children on their hands he reacted angrily.
He said "One does not need to support people who not only kill their enemies, but open up their bodies, eat their intestines in front of the public and cameras. Are these the people you want to support? Is it them you want to supply with weapons? Then this probably has little relation to humanitarian values that have been preached in Europe for hundreds of years."
The summit is taking place amid a security operation of unprecedented scale, with some 8,000 police officers - 3,600 of them borrowed from forces in England, Scotland and Wales - on patrol.
Vehicle checkpoints are back on the streets, there is a no-fly-zone over the venue and an "armada" of security vessels is patrolling a secure stretch of Lough Erne.
Police have warned of significant disruption. Eight world leaders and their entourages will be on the move. Some 8,000 students sitting exams were advised to travel to school early.
