The call came after UN investigators reported on atrocities committed inside the country, where rights groups say more than 15,000 people have been killed since an uprising began against President Bashar al-Assad.
Monitors tracking the violence said the last week had been the bloodiest since the fighting started, as a deadly attack on a pro-government television station sparked international outrage.
Annan's proposed interim authority would exclude officials whose presence might jeopardise the transition or undermine efforts to bring reconciliation, according to a summary given by one UN diplomat.
The major powers -- the US, Britain, France, China and Russia, a key Assad ally -- generally back the plan that will be discussed at a meeting of foreign ministers Annan has convened in Geneva on Saturday, the diplomats said.
"The language of Annan's plan suggests that Assad could be excluded but also that certain opposition figures could be ruled out," said a second UN diplomat, while stressing that there was nothing there that automatically excluded him.
"Russia's acceptance of this plan could be a new sign that it is ready to let Assad go," said the diplomat.
But Russia's UN ambassador Vitaly Churkin said there was no guarantee that Annan's document would be agreed to in Geneva.
"Whatever Kofi Annan is going to prepare is going to be basis of discussion for the ministers," he told reporters.
Turkey, Qatar, Kuwait and Iraq will also be at the meeting, but Iran and Saudi Arabia were not invited.
Both Annan and UN leader Ban Ki-moon had wanted Iran to take part in the talks, as had Russia. But the United States and European nations had opposed the Islamic state's involvement.
Speaking in Helsinki on Wednesday, US Secretary of State Hillary Clinton said she had been in regular contact with Annan over his transition plan, without saying what it contained.
"We think it embodies the principles needed for any political transition in Syria that could lead to a peaceful, democratic and representative outcome reflecting the will of the Syrian people," she added.
In Geneva, United Nations investigators who had visited Syria reported back to a meeting of the UN Human Rights Council Wednesday on the toll the 15-month conflict was taking on the country.
Attacks were becoming increasingly sectarian in nature, the commission of inquiry (CoI) report said.
"Where previously victims were targeted on the basis of their being pro- or anti-government... a growing number of incidents... appear to have been targeted because of their religious affiliation," said the report.
That prompted a walk-out by the Syrian delegation, with ambassador Faisal Khabbaz Hamoui denouncing a war of disinformation against Syria.
The report also gave its findings on the the massacre in the central town of Houla where at least 108 people were killed over a 24-hour period on May 25-26.
"The CoI is unable to determine the identity of the perpetrators at this time, nevertheless the CoI considers that forces loyal to the government may have been responsible for many of the deaths," it said.
It did not however rule out the possibility that anti-government forces might have been involved, as the Damascus regime has alleged.
The commission also said snipers had killed a large number of children, and it spoke of multiple reports of rape and sexual assault after government forces entered the Baba Amr neighbourhood of Homs in February.
Wednesday's attack on the offices of Al-Ikhbariya television near Damascus killed three journalists and four security staff, state media reported.
"The terrorist groups stormed the offices of Al-Ikhbariya, planted explosives in the studios and blew them up along with the equipment," Information Minister Omran al-Zohbi said from the scene.
Live footage broadcast by state television showed extensive damage to the studios. Al-Ikhbariya itself remained on air despite the assault.
"They carried out the worst massacre against the media, executing journalists and security staff," Zohbi said. A number of staff had also been kidnapped, he added.
The US condemned the attack and rights group including Amnesty International condemned the attack.
"We condemn all acts of violence including those targeting pro-regime elements," White House spokesman Jay Carney said.
Amnesty International's Middle East and North Africa deputy director Ann Harrison said: "Even a media organisation engaged in propaganda is still a civilian object, so it and those working for it must never be deliberately targeted."
At least 82 more people were killed inside Syria Wednesday, including the victims of the attack on the television station and 40 other civilians, the Syrian Observatory for Human Rights reported.
Observatory chief Rami Abdel Rahman told AFP that 916 people had been killed between June 20 and 26, making the period the bloodiest week of the Syrian Revolution.
More than 15,800 people have been killed in the uprising which erupted in March 2011, the Observatory said, 4,681 of them since a ceasefire previously proposed by Annan was supposed to take effect on April 12.