The main point of the summit "is to fire up our economies and drive growth and prosperity around the world," Cameron told a news conference. "And there is no more powerful way of doing this than boosting trade."
Besides Cameron, U.S. President Barack Obama, European Commission President Jose Manuel Barroso and European Council President Herman Van Rompuy, commented on the proposed Transatlantic Trade and Investment Partnership talks that are to begin in earnest in Washington July 8.
A trade deal "could add as much as a $157 billion to the EU economy, $125 billion to the U.S. economy and as much as $133.4 billion to the rest of the world," Cameron said.
Obama characterized the trade talks as "groundbreaking."
"It would increase exports, decrease barriers to trade and investment," he said, noting the deal would support "hundreds of thousands of jobs on both sides of the ocean."
Obama said trade between the European Union and the United States "makes up nearly half of global gross domestic product."
"We trade about $1 trillion in goods and services each year. We invest nearly $4 trillion in each other's economies. And all that supports around 13 million jobs on both sides of the Atlantic," he said.
Soon after their arrival in Belfast, President Obama and first lady Michelle Obama spoke at a town hall meeting of young people at the Belfast Waterfront.
Obama told the young people in the audience they were providing a "blueprint" to other countries afflicted by violence, that a lasting peace was possible and that "hope is contagious."
"[You] set an example for those who seek a peace of their own," Obama said. "And they're groping to find a way to discover how to move beyond the heavy hand of history, to put aside the violence. They're studying what you're doing. And they're wondering, perhaps if Northern Ireland can achieve peace, we can, too. You're their blueprint to follow. You're their proof of what is possible -- because hope is contagious."
Russian President Vladimir Putin scolded the West before the G8 sessions for for considering arming Syrian rebels who he said eat human organs.
Putin said his country, by contrast, supplied weapons to Syrian President Bashar Assad's "legitimate government" -- a government Canadian Prime Minister Stephen Harper said was run by "thugs."
Obama was to meet privately with Putin.
Putin spoke at a news conference with British Prime Minister David Cameron.
Putin attacked the decision by Obama last week to deliver arms to Syrian rebels to tilt the civil war's balance away from the regime and try to force it to the negotiating table.
Moscow is the main diplomatic backer and arms supplier for Assad, whose efforts to defeat opposition forces seeking his ouster have contributed to a death toll the United Nations said last week was nearly 93,000.
At one point in the news conference, Putin referred to a disturbing video posted on the Internet last month of a breakaway Syrian rebel fighter biting into an organ cut out of a regime soldier's corpse.
The fighter, Khalid al-Hamad, who goes by the war name Abu Sakkar, told Time magazine shortly afterward he considered the act just punishment for the soldier's torture and sexual abuse of a naked mother and her two naked daughters before he was killed.
Putin said Sunday: "I think you will not deny that one does not really 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 who you want to supply with weapons? Then this probably has little relation to the humanitarian values that have been preached in Europe for hundreds of years."
Harper, speaking at a reporters briefing in Dublin, Ireland, before arriving in Northern Ireland, castigated Putin for supporting the Assad regime and even suggested Putin didn't have the same status at the table as the seven leaders of some of the most advanced Western democracies.
"I don't think we should fool ourselves," Harper said. "This is the G7 plus one. Let's be blunt. That's what this is -- the G7 plus one."
The G7 -- Britain, Canada, France, Germany, Italy, Japan and the United States -- has met for decades as a way for like-minded leaders to talk candidly about global problems. Russia was invited to join in 1997, creating the G8.
"Mr. Putin and his government are supporting the thugs of the Assad regime for their own reasons, which I do not think are justifiable, and Mr. Putin knows my view on that." Harper told reporters.