The conflict in Syria was set to dominate the G8 summit starting in Northern Ireland on Monday, with Western leaders upping pressure on Russia to back away from its support for President Bashar al-Assad.
Increasingly isolated from fellow G8 leaders, Russian President Vladimir Putin was likely to get a frosty reception at the two-day summit in the lake-fringed Lough Erne golf resort.
Planned face-to-face talks between Putin and US President Barack Obama were set to be especially prickly, with both leaders now offering military support to opposing sides in the war.
In Moscow on Monday, the foreign ministry said Russia would not permit a no-fly zone to be imposed over Syria, an idea that media reports say is being mooted in Washington.
Summit host Prime Minister David Cameron, who had initially hoped for talks to focus on finance and trade, said Monday his priority was to ensure a peace conference on the Syria conflict takes place later this year.
"What we do need to do is bring about this peace conference and this transition, so that people in Syria can have a government that represents them, rather than a government that's trying to butcher them," Cameron said in a round of television interviews.
Washington and Moscow have been pushing for Syria's regime and the opposition to hold peace talks in Geneva, but the efforts have so far been fruitless.
In his talks with Putin, Obama will emphasise that Washington wants to keep alive the proposed peace conference in Geneva, which appears to be slipping down the list of priorities.
But on the eve of the summit Canadian Prime Minister Stephen Harper made it clear that few G8 leaders expected Putin to change his position.
"I don't think we should fool ourselves. This is the G7 plus one," Harper told reporters in Dublin. "Unless there's a big shift of position on his part, we're not going to get a common position with him at the G8."
French President Francois Hollande, due for private talks with Putin later, also criticised Russia for arming Syria's regime.
"How can we accept that Russia continues to deliver arms to Bashar al-Assad's regime while the opposition receives very few and is being massacred?" Hollande told journalists.
In a bright spot just before the formal start of the summit, Cameron, Obama and EU leaders announced the formal start of negotiations on a vast free trade pact between the European Union and the United States.
"We are talking about what could be the biggest bilateral deal in history," Cameron said, before Obama said the first round of negotiations would take place in Washington next month.
EU nations agreed to go ahead with the talks after late-night discussions in Luxembourg on Friday to convince France that its prized cultural industries would not be under threat from the pact.
Officials have said the deal could be worth more than 200 billion euros ($265 billion) annually to the European and US economies.
Obama went straight from landing in Belfast shortly to give a speech to 2,000 mostly young people, in which he urged them to preserve Northern Ireland's hard-won peace.
Roads in and out of Belfast were closed, hundreds of police 4x4s lined the streets and a police helicopter patrolled the skies over the city.
Thousands of extra police officers were deployed to the province ahead of the summit, in the biggest security operation in Northern Ireland's troubled history.
Cameron will host Obama, Putin, Hollande, Harper, German Chancellor Angela Merkel, Japanese Prime Minister Shinzo Abe and Italian Prime Minister Enrico Letta.
The British premier may face some uncomfortable questions after documents leaked by US former spy Edward Snowden appeared to show that Britain spied on foreign delegates at the 2009 London G20 meetings.
Among the officials targeted were delegates from NATO ally Turkey and from fellow Commonwealth state South Africa, according to British newspaper The Guardian.
Asked whether he could guarantee his guests that no similar operation was in place as they gathered at Lough Erne, Cameron would not be drawn.
"We never comment on security or intelligence issues and I am not about to start now," he said.
The British hosts of the gathering also want to forge consensus on cracking down on tax evasion and making multinational companies more transparent.