President Obama and his Russian counterpart, Vladi­mir Putin, failed to resolve on Monday their significant differences over how to bring about an end to Syria’s civil war, as each leader steps up military support for opposite sides in the worsening conflict.

Meeting for two hours on the sidelines of the Group of Eight summit, Obama and Putin discussed shared economic interests, the recent Iranian elections and global security issues that have put the leaders at odds in the past.

Syria’s civil war was chief among them. Sitting stiffly in side-by-side chairs, Obama and Putin each indicated that they still disagree over the preferred outcome of the war, including on the future of President Bashar al-Assad and the goals of the armed rebellion.

“Our opinions do not coincide,” Putin said. “But all of us have the intention to stop the violence in Syria.”

Obama, speaking next, confirmed that “we do have differing perspectives” on the war and how to resolve it through negotiations that have yet to take shape.

Obama has demanded that Assad relinquish power as part of any negotiated peace settlement, a condition Putin rejects. Russia is Assad’s principal weapons supplier, and the Obama administration is about to begin arming rebels on the other side of the civil war that has killed an estimated 93,000 people over the past two years, according to U.N. estimates.

Little is known about some of the groups fighting Assad. There have been reports, supported by video, of atrocities carried out by some rebel factions.

France and the United Kingdom, though, successfully sought to lift an European embargo on arms deliveries to the rebels. Obama, after months of deliberation, has decided to supply light weapons and ammunition to opposition forces.

But Putin warned that the move was dangerous, saying after a meeting Sunday with British Prime Minister David Cameron that arming the rebels “has little relation to humanitarian values that have been preached in Europe for hundreds of years.”

Western diplomats had given Obama little chance of changing Putin’s opinion on Syria here. But his inability to do so still posed an early setback for Obama on a three-day swing through Europe, his first to the continent since 2011.

This time he is facing rising skepticism in Europe over his expansion of drone warfare, recent disclosures about the National Security Agency’s vast data-collecting efforts and his delay in more aggressively supporting Syria’s beleaguered rebel forces.

Obama began meeting near this picturesque town Monday with G-8 leaders, hoping to mend fences and achieve a broader international consensus on how to improve the lagging global economy.

Hours before the summit, Obama and European leaders announced the start of negotiations to forge a new trade agreement between the United States and the 27-nation European bloc.

The Transatlantic Trade and Investment Partnership would create what Cameron called “the biggest bilateral trade deal in history,” although talks are expected to be complicated despite urgency on both sides of the Atlantic to boost economic growth. The first round of negotiations will be held next month in Washington.

“There are going to be sensitivities on both sides, there are going to be politics on both sides,” Obama said. “But I’m confident we can get it done.”

Shadowing the summit’s start Monday were new revelations that British and U.S. spy agencies monitored the e-mails and phone calls of foreign dignitaries at two international summits in London in 2009.

The Guardian newspaper, citing documents it received from former National Security Agency contractor Edward J. Snowden, revealed the spying as the G-8 leaders gathered at the Lough Erne resort for a day and half of meetings.

The disclosure follows recent reports in The Washington Post and the Guardian , also based on documents provided by Snowden, that disclosed widespread U.S. surveillance of phone and Internet use by ordinary citizens to detect patterns that could indicate terrorist activity.

The latest revelations by the Guardian focused on two London summits in 2009 hosted by then-Prime Minister Gordon Brown. British intelligence agents, the newspaper said, went as far as setting up fake Internet cafés and tapping into cellular networks of diplomats and foreign officials.

On Monday, Cameron told Britain’s Sky News that “we never comment on security or intelligence issues and I am not about to start now.” White House officials also declined to comment.

Obama began the day in Belfast, a city once defined by conflict and now living in uneasy peace.

There he urged Northern Ireland’s youngest generations to reject the temptation of violence as, he said, technology and citizen activism are breaking down barriers in much of the world.

History colored Obama’s remarks, which he delivered along the city’s thriving waterfront 15 years after a peace agreement ended decades of sectarian conflict between Catholic Republicans seeking alliance with their southern neighbor and Protestants loyal to the United Kingdom.

Under a drizzling sky, teenagers in school blazers and ties lined up hours before the event outside Waterfront Hall for a chance to see Obama on his first visit to Northern Ireland. Obama told them to defend their fragile peace and to count on the United States when the Good Friday Agreement, brokered by former U.S. senator George Mitchell (D-Maine), is tested as it has been this year.

“The terms of peace may be negotiated by leaders, but the fate of peace is up to you,” Obama told the audience.

Anthony Faiola in London contributed to this report.