Cameron talks up free trade deal
Mr Cameron spoke privately with Barack Obama - who has said he may arm the rebels seeking to oust the regime of Bashar Assad - ahead of the US president's talks with Russia's 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 on Sunday when the Russian president warned the international community to be wary of arming militants who "eat the organs" of their enemies.
Mr Cameron said 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.
But the Prime Minister was also fighting to keep the world's attentions focused on the economic priorities which he has set for the two-day G8 summit in Lough Erne, particularly the EU/US pact which he said would be "the biggest bilateral trade deal in history".
"The whole point of this meeting in Lough Erne is to fire up our economies and drive growth and prosperity around the world, to do things that make a real difference to people's lives," said the PM, in shirt sleeves and tieless after completing talks with Mr Obama and European Union (EU) leaders.
"There is no more powerful way to do that than by boosting trade, and there is no better way than by launching these negotiations on a landmark deal between the EU and the USA, a deal that could add as much as £100 billion to the EU economy, £80 billion to the US economy and as much as £85 billion to the rest of the world. They mean extra jobs, more choice and lower prices in our shops."
Mr Obama promised to make the trade talks, which start next month in Washington and are expected to last up to two years, a priority of his remaining term in office. "America and Europe have done extraordinary things together before and I believe we can forge an economic alliance as strong as our diplomatic and security alliances, which of course have been the most powerful in history," he said.
Divisions with Russia over Syria were underlined as Mr Obama conceded there were "differing perspectives" between Washington and Moscow on the issue. After talks with Mr Putin, the US leader said: "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."