David Cameron is unlikely to change his stance on backing 'moderate' Syrian rebels but there is genuine uncertainty as to Obama's plans.
David Cameron admitted there were 'big differences' between the UK and Russia on the Syria conflict, but insisted they could be overcome
The Prime Minister joined talks with Britain's overseas territories on measures to clamp down on tax evasion through offshore territories.
David Cameron is not expecting others to follow suit in making terror ransoms illegal but has said he hopes to secure a "tangible agreement" to frustrate the lucrative practice.
His drive will form the centrepiece of a session of talks focused on joint action to tackle the terror threat as he reconvenes fellow leaders for the second day of the UK-hosted summit in Northern Ireland.
David Cameron is expected to urge all G8 leaders to commit their countries not to pay terror ransoms in an international effort to cut off extremists' funding.
Up to 70 million US dollars (£45 million) is estimated to have been paid to secure the release of Western captives in the last three years alone - an average 2.5 million US dollars (£1.59 million) per victim.
Much of that is believed to have ended up in the coffers of terror groups including al Qaida and its affiliates and the Taliban.
The UK outlaws such payments but other countries - including some within the group of leading industrialised nations - continue to meet the demands to the frustration of non-payers.
Sitting side by side after two hours of talks, President Obama and Russia's President Vladimir Putin were as far apart as ever on Syria tonight.
After their meeting at the world leaders' summit in Northern Ireland, Obama said that they did agree that the violence in Syria had to stop, but had different perspectives.
Putin, Syria's ally, put it more bluntly - a difference of opinion, he called it.
ITV News' political editor Tom Bradby reports:
At the last G8 Summit, just over a year ago, the death toll from the fighting in Syria stood at 10,000. Since then it has gone up tenfold.
ITV News' Paul Davies reports:
The White House has announced that the US will provide $300 million in humanitarian aid for Syrians, split between Syria and neighbouring countries dealing with refugees.
Syria was not the only issue discussed at today's G8 talks, but it was certainly the most pressing.
The body language between the Russian and American presidents was terrible, and it was evident that some pretty harsh words were exchanged.
Having said that, I am told by officials tonight at dinner there was a slightly better mood and that all sides are very keen to push for the idea of another peace conference in Geneva.
Given that body language and how far apart the Russians and Americans seem to be, you do have to wonder what the chances of success at any such conference would really be.
The critical thing here is that we know that the Americans are intending to arm the rebels - Barack Obama has said so.
What he hasn't done is fill in any of the detail. What is expected here over the next 24 hours or so is we'll begin to get that detail both from the Americans and from David Cameron.
Protesters broke through a security fence outside the G8 summit in Loch Erne, Enniskillen.
After crossing a barbed wire barrier protesters crossed a field and approached a police line before returning.
Barack Obama today tried to lighten the mood at the end of his talks with Russian leader Vladimir Putin, by bringing up the topic of judo.
However, Putin, who is a black belt in the martial art, replied that the US president was simply trying to get him to relax.
Syrian President Bashar al-Assad has told a German newspaper that there is a risk of terrorists "returning to Europe with fighting experience", and disputed the US assertion that his forces had used chemical weapons.
Speaking in an interview with Frankfurter Allgemeine Runschau, he said: "If the Europeans ship weapons, Europe's backyard becomes a terrorists' place, and Europe will pay a price for it."
"Terrorists will return to Europe with fighting experience and extremist ideologies.
"That's why it is not logical to use chemical weapons to kill a number of people that can be achieved through conventional weapons."
"If Paris, London and Washington had only one piece of evidence backing up their allegations, they would have unveiled it to the world."
The US and Russia have released a joint statement regarding counter-terrorism measures.
– Joint statement from the US and Russia
The joint efforts of the United States and Russia, including in the context of the Counterterrorism Working Group of the Bilateral Presidential Commission, are focused on preventing weapons of mass destruction from falling into the hands of terrorists, and halting the funding of terrorist activities, the recruitment and training of those who commit terrorist acts, and the actions of lone terrorists.