By Andrew Osborn and Oliver Holmes
ENNISKILLEN, Northern Ireland/BEIRUT (Reuters) - Differences between Russia and the West mean an international peace conference on Syria is now unlikely before August, a source at a meeting of Group of Eight leaders said on Tuesday as surging government forces brought heavy fighting to Aleppo.
World leaders called for peace talks to be held as soon as possible to end the war in Syria but made no mention of a date for the international conference, which had been due to be held in Geneva next month.
In Aleppo, several fronts in Syria's biggest city that had been relatively quiet for some time were now experiencing heavy fighting as government troops have gained ground this month, according to an opposition monitoring group.
Russian President Vladimir Putin, a backer of Bashar al-Assad, appeared isolated at a summit of the Group of Eight in Northern Ireland, resisting attempts to persuade him to moderate his support for the Syrian president.
U.S. President Barack Obama, moving since last week towards arming the rebels fighting to oust Assad, said it was important to build a strong opposition that could function after the Syrian leader loses power.
"We remain committed to achieving a political solution to the crisis based on a vision for a united, inclusive and democratic Syria," G8 leaders said, according to a copy of the final communique seen by Reuters.
Peace talks envisaged for July were unlikely to be held before August, according to one source at the summit.
The communique made no mention of Assad, who Western leaders have said in the past said must step down as part of a resolution. Russia had said that any such reference to Assad's fate in the document would not be acceptable.
G8 leaders also called on the Syrian authorities and the opposition to commit to destroying all organizations affiliated with al Qaeda. Members of the militant group and allied Islamist fighters have been in action alongside the rebels.
Obama and his allies want Assad to cede power while Putin, whose rhetoric has become increasingly anti-Western since he was re-elected last year, believes that would be disastrous at a time when no clear transition plan exists.
Russia, which has given Assad diplomatic cover as well as weapons, urged the West to think "three or four times" before going ahead with plans to arm the rebels.
Moscow could not accept that Assad had used chemical weapons against the rebels, an allegation that had tipped Washington's hand in deciding to arm the anti-Assad fighters, Russian Deputy Foreign Minister Sergei Ryabkov said.
FIGHTING IN NORTH
As peace efforts have faltered and arms have flowed into rebel hands, heavy fighting on the northern front lines in and around Aleppo has resumed. Government forces are seeking to build on battlefield gains further south.
Those backing the rebels - including Britain, France, Turkey and Arab countries as well as the United States - were driven to intensify support in recent weeks to rescue the rebellion after Assad's forces scored important military gains.
Just a few months ago, Western countries thought Assad's days were numbered. But last month he received the open support of thousands of fighters from Hezbollah, the powerful Iran-backed Shi'ite militia in neighboring Lebanon, which helped him capture the strategic town of Qusair from the rebels this month.
Rami Abdelrahman, head of the anti-Assad Syrian Observatory for Human Rights, said there were clashes in the eastern Sakhour neighborhood of Aleppo as well as in the Old City, which sits between government and rebel-held territory.
An opposition activist said rebels and government forces were fighting in the alleyways of Aleppo's historic Old City.
The Observatory also reported clashes in Damascus, Homs, Hama, Deraa and the eastern city of Deir al-Zor.
In Idlib province, in the north west, a rocket hit the house of a prominent religious figure who is known to support pro-Assad militia, killing 20 people, the Observatory said.
In Lebanon, militants supporting opposing sides in Syria's civil war clashed in the southern city of Sidon on Tuesday, killing one person, a security source said, in a city where divisions have been simmering for months.
The recent upsurge of fighting has turned Syria's war into a sectarian conflict between Sunni Muslim rebels and members of Assad's Alwaite sect, an offshoot of Shi'ite Islam, and their Shi'ite Hezbollah allies.
Lebanon on Tuesday accused Assad's forces of driving Sunni Muslims across the border into its territory.
Syrian forces had committed what was "tantamount to ethnic cleansing next to the Syrian-Lebanese border", Wael Abu Faour, the Lebanese caretaker minister for social affairs, told Reuters.
"(Assad) is trying to displace all the Sunnis to Lebanon and this is why I expect to have more displaced people," he said.
The United Nations says 93,000 people have been killed in Syria and 1.6 million Syrians have fled abroad. Lebanon, the smallest of Syria's neighbors, has taken in more than half a million Syrian refugees.
(Writing by Giles Elgood; Editing by Peter Graff)