The opposition Syrian National Council (SNC) will not accept a unity government led by a member of the regime, spokeswoman Bassma Kodmani said.

"There was never any question of a national unity government led by a member of the regime," Kodmani told AFP.

"There is of course a question of a transitional government, but of the opposition, not led by a member of the state."

An emailed statement from the SNC also ruled out a unity government led by a member of President Bashar Assad's regime.

The Arab League on Monday called on Assad to swiftly step aside in order to end the fighting that has swept across the country.

The Syrian regime has rejected all Arab League calls for a transition plan that would see Assad leave power.

Meanwhile, Syria's rebels accused Bashar Assad of moving chemical weapons to the country's borders, a day after his beleaguered regime said it would use its stockpiles if attacked.

"According to our information, the regime began moving its stocks of weapons of mass destruction several months ago... with the goal of putting pressure on the region and the international community," Free Syrian Army said in a statement.

At a Damascus news conference on Monday, foreign ministry spokesman Jihad Makdissi acknowledged that Syria has chemical weapons and said the regime would use them if attacked by outsiders.

US President Barack Obama warned Assad not to make the "tragic mistake" of unleashing chemical weapons.

"Given the regime's stockpile of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching," Obama said in Nevada.

"They will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons," he added.

Analysts said they doubted whether in the current conflict Assad's regime would resort to using a chemical weapons stockpile that was developed as a counterbalance to Israel's presumed nuclear arsenal.

"It's not at all in the interest of the Syrian army to use these weapons — there is no tactical interest given the nature of the current conflict which is mainly urban," said Olivier Lepick, specialist in chemical weaponry at the Foundation for Strategic Research in Paris.

"The regime knows that if it did so it would lose the support of China and Russia."