G8 leaders called Tuesday for a peace conference on Syria to be held as soon as possible but deep divisions remained as Russia stood by its embattled Middle East ally.
At the end of two days of tough talks in Northern Ireland, the leaders agreed to push for a transitional government in Syria that could include members of President Bashar al-Assad's regime who switched sides.
The Syria crisis overshadowed a deal by the world's leading industrialised nations gathered on the picturesque banks of Lough Erne to crack down on tax evasion and share more cross-border financial information.
British Prime Minister David Cameron insisted the leaders had forged a strong agreement on Syria despite a split with Russian President Vladimir Putin, but their closing statement was short on concrete steps.
Cameron, the summit host, said Assad could not join a transitional administration after the deaths of 93,000 people and what Western nations say is the use of chemical weapons.
"As for the transition, look I think it unthinkable that President Assad can play any part in the future government of his country, he has blood on his hands, he has used chemical weapons," he said.
The G8 communique pointedly made no reference to him however in an apparent concession to Moscow, Assad's chief arms supplier.
The statement said only that the transitional body should be "formed by mutual consent".
The G8 harked back to the chaos after the 2003 invasion of Iraq, saying that Syrian military and security services "must be preserved and restored" in a future set-up.
The leaders did not suggest a date for the proposed Syria peace talks, which were supposed to take place this month in Geneva to follow up on a similar meeting last year but have already been delayed.
They did however urge Syria to admit chemical weapons investigators and say they were "deeply concerned" by the threat of Islamic extremism among the rebels.
The Syria conflict has sparked fears of a new cold war with Washington saying last week that it would start arming the rebels against the Russian-backed Syrian regime.
Putin, who had an icy confrontation with US President Barack Obama on Monday, was in defiant mood after the summit, saying that Russia would not rule out new arms supplies.
"We are supplying arms to the legitimate government in accordance with legal contracts," Putin told a news conference.
He denied however that he had felt frozen out of the summit by the G8, to which Russia was only admitted in 1998 and said that "not a single time did it happen that Russia was left alone in defending its approach to the solution of the Syrian problem".
British officials pointed to Cameron's efforts to win over Putin in the run-up to the summit, including travelling for talks with the Russian president in Sochi in May.
The Black Sea resort will host the next G8 summit on June 4-5, 2014.
The G8 nations pledged almost $1.5 billion (1.1 billion euros) in humanitarian aid for refugees inside and outside Syria, including $300 million from the United States and 200 million euros from Germany.
French President Francois Hollande meanwhile said that Iranian president-elect Hassan Rowhani would be welcome at the Syria peace talks "if he can be useful."
The G8 leaders were more united on tax, vowing concrete steps to target not only illegal tax evasion but also tax avoidance by multinational companies that costs taxpayers billions in lost revenues.
And they agreed to stamp out the payment of ransoms for hostages kidnapped by "terrorists", and called on companies to follow their lead in refusing to pay for the release of their employees.
On tax, Cameron heralded a commitment in the declaration to fight the "scourge" of tax evasion and to promote corporate transparency.
"Countries should change rules that let companies shift their profits across borders to avoid taxes, and multinationals should report to tax authorities what tax they pay where," the G8 said.
But activists said the deal came up short.
Alex Wilks, campaign director at global civic organisation Avaaz said opposition from Canada and Germany "blocked the strong deal the world demanded."
Hollande said the deal was a "big step forward" but admitted: "We wanted to go even further."
The summit also saw the launch of formal negotiations on a vast trade pact between the United States and the European Union.
The meeting was guarded by 8,000 police officers in the biggest security operation ever mounted in Northern Ireland's troubled history, but protesters were thin on the ground.
The G8 brings together Britain, Canada, France, Germany, Italy, Japan, Russia and the United States.