"I can only warn the Syrian regime: a use of chemical weapons would be totally unacceptable," German Foreign Minister Guido Westerwelle said ahead of the gathering of his Nato counterparts in Brussels.

"Whoever even considers this should know that the world would bring them to justice. We demand an immediate end to the violence in Syria and for power to be handed over to institutions preparing the transition."

Nato foreign ministers were expected to approve Turkey's request for Patriots, after US President Barack Obama issued a sharp warning to Damascus against the use of chemical weapons.

A US official has told AFP that Syria had begun mixing chemicals that could be used to make sarin, a deadly nerve agent, while CNN reported Damascus could use the gas in a limited artillery attack on advancing rebels.