Syrian opposition groups will reject a political transition plan proposed by peace envoy Kofi Annan unless it explicitly requires President Bashar al-Assad to step down before a unity government is formed, a senior opposition official said on Thursday.

Diplomatic sources at the United Nations said Annan's proposal, aimed at ending the 16-month conflict in Syria, does not stipulate Assad's resignation although it does say the unity government could not include figures who jeopardise stability.

"The proposal is still murky to us but I can tell you that if it does not clearly state that Assad must step down, it will be unacceptable to us," said Samir Nashar, an executive member of the international Syrian National Council.

Annan's transition proposal is one of the main topics that Russia, the other four permanent U.N. Security Council members and key players in the Middle East will discuss at a meeting in Geneva on Saturday, according to United Nations diplomats.

To win U.N. backing the plan must have the support of Syria's powerful ally Russia, which has so far rebuffed Western attempts to force Assad to cede power as rank interference in the country's sovereignty.

For the opposition, Assad's departure is imperative.

"If the proposal said Assad must step down, then the idea of allowing other members of the current government to participate could be open to discussion," Nashar said.

But rebel fighters locked in the war to topple Assad said there was no part of the plan they could accept, and they had lost patience with U.N. envoy Annan's peace-making efforts.

"This is just a new labyrinth. It is new silliness for us to get lost in and haggle over who can participate and who can't," said Ahmed, a Free Syrian Army (FSA) fighter in Homs, epicentre of the revolt against four decades of Assad family rule in which the more than 10,000 people have been killed, by a UN count.

A member of the rebel group in Damascus suburb was also dismissive.

"I'll be direct. The FSA is doing its work, and it is not looking to the outside world. We don't want a transitional government unless it is the one formed by rebel military councils. The world is conspiring against the Syrian revolution," he said.

In April, Annan tried to implement a ceasefire to quell violence before embarking on peace talks. But the truce failed to take hold.

Diplomatic sources at the United Nations said the plan Annan will now pitch on Saturday aims to start the political process without waiting for a ceasefire.

"I'm against the plan," rebel fighter Ahmed said. "And I'm against Annan. How many plans has he offered? All they do is give more time for the killing to continue."

UN diplomats said on Wednesday Russia and other big powers had told Annan they support his idea of a Syrian national unity cabinet that could include government and opposition members but would exclude those whose participation would undermine it.

One diplomat summarized Annan's proposal, saying it called for an irreversible settlement with clear transition steps in a fixed timeline.

"These include establishing a transitional national unity government to create a neutral backdrop for transition," the diplomat said. "It could comprise present government members, opposition and others, but would need to exclude those whose continued participation or presence would jeopardise the transition's credibility, or harm prospects for reconciliation and stability."

The diplomat added that the idea of excluding certain people was clearly referring to President al-Assad.-Reuters