Syrian National Council chief Burhan Ghalioun said he will step down to avert divisions within the opposition bloc, after activists on the ground accused him of monopolising power.
"I will not allow myself to be the candidate of division, I am not attached to a position, so I announce that I will step down after a new candidate has been chosen, either by consensus or through new elections," said the Paris-based academic.
He spoke as Syrian forces launched a blistering assault on the rebel stronghold of Rastan in central Homs province, in a new bid to overrun one of the major opposition holdouts against President Bashar al-Assad's regime.
There were also more deaths and anti-regime protests, as UN observers tasked with monitoring a tenuous UN-backed ceasefire visited the Aleppo University campus, activists said.
He said he would remain an SNC member "hand-in-hand with the young people who struggle, the young people of the revolution of dignity and freedom, until victory," while urging all opposition groups to unite ranks.
Hours earlier the Local Coordination Committees, a network of activists on the ground, threatened to pull out of the SNC over its lack of collaboration with activists in Syria and "monopolisation" of power.
"The deteriorating situation in the SNC is an impetus for us to take actions, which could begin with a freeze (of LCC membership in the SNC) and end with a withdrawal if errors are not solved and demands for reform go unmet," the LCC said.
It pointed to "a total absence of consensus between the SNC's vision and that of the revolutionaries" and charged that influential SNC members were monopolising power and marginalising most of the LCC representatives.
The LCC also criticised the SNC over the strong influence that Syria's Muslim Brotherhood wields over the coalition.
Meanwhile, Rami Abdel Rahman of the Syrian Observatory for Human Rights, said at least 30 shells smashed into Rastan in a 10-minute period after midnight, and urged UN observers monitoring a shaky truce to immediately rush to the town.
"The army is trying to gradually destroy Rastan," Abdel Rahman told AFP.
The Britain-based Observatory also reported that five civilians, members of the same family and including two children, were killed Thursday when a mortar shell slammed into their home in Douma, near Damascus.
And in northern Syrian thousands of students from Aleppo University protested on campus, calling for the fall of the regime, as UN observers visited, activists said.
A spokesman for a group of activists in Aleppo, Mohammed Halabi, told AFP in Beirut by telephone:
"Thousands of students from various faculties came out of their classes when the UN observers arrived and shouted slogans calling for the fall of the regime."
They "also called for the arming of the (rebel) Syrian Free Army)," he said in a telephone call.
Activists also posted videotapes showing students insulting President Bashar al-Assad and chanting for freedom in Syria.
Assad, in an interview with Russian TV, accused the West on Wednesday of ignoring violence by "terrorists" and said he would demand an explanation from Annan when he visits Damascus this month.
He also denounced the armed opposition as a gang of "criminals" who he said contained religious extremists, including members of Al-Qaeda.
Russia, a key ally of Assad's regime, cautioned Western powers against launching "hasty" wars that could lead to the rise of radical Islamist factions and all-out regional war.
"The consequence of hasty military operations in foreign states usually means that radicals come to power," said Dmitry Medvedev, Russia's premier and former president.
"And sometimes these actions -- which undermine state sovereignty -- could result in a fully fledged regional war," he said, in clear reference to Moscow's current standoff with the West over Syria.
The prime minister of Qatar, which has in the past called for arming the Syrian opposition, accused Damascus of continuing to kill civilians.
"The killing of civilians is ongoing there regardless of the international will to stop this bloodshed," said Sheikh Hamad bin Jassem al-Thani during a visit to Bulgaria.
"UN negotiations are underway but the killings continue. This is intolerable," he said.
More than 12,000 people, the majority of them civilians, have died since the Syrian uprising began, according to the Observatory, including more than 900 killed since the putative truce came into effect.
The UN mission in Syria says it now has 236 military observers in the country.© ANP/AFP