NATO foreign ministers are expected on Tuesday to approve Turkey's request for Patriot anti-missile systems to bolster its defense against possible strikes from neighboring Syria.

Ankara, which has been highly supportive of the Syrian opposition, wants the Patriots to defend against possible retaliatory attacks by Syrian missiles.

The Syria crisis is expected to dominate the two-day NATO meeting in Brussels, with a decision on Turkey's request at the top of the agenda.

Both NATO and Turkey insist that the deployment of the US-made surface-to-air Patriot missiles is a purely defensive move and alliance diplomats pressed this point on Monday.

But Russian President Vladimir Putin warned on Monday that any deployment of the missiles would only add to tensions and possibly widen the conflict.

"Creating additional capabilities on the border does not defuse the situation but on the contrary exacerbates it," Putin said after meeting Turkish Prime Minister Recep Tayyip Erdogan in Istanbul.

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

But the Syrian Foreign Ministry said Damascus will never use chemical weapons on its people.

China on Tuesday reaffirmed its stance of banning weapons of mass destruction.

"China has always advocated a comprehensive ban on all WMDs and supported the purpose and aim of the Chemical Weapons Convention," Foreign Ministry spokesman Hong Lei said on Tuesday.

"China opposes the development and use of chemical weapons and maintains the enhanced universality of the Chemical Weapons Convention," he said.

Hong's comments came after US President Barack Obama on Monday said Syrian President Bashar al-Assad's use of chemical weapons is totally unacceptable.

"I want to make it absolutely clear to Assad and those under his command, the world is watching, the use of chemical weapons is and would be totally unacceptable," Obama said.

"If you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable," he added.

Earlier, US Secretary of State Hillary Clinton said chemical weapons were "a red line for the US", adding that Washington was "certainly planning to take action if that eventuality were to occur".

The Syrian issue triggered the deteriorated ties between Russia and Turkey, and Putin visited Turkey to improve relations.

Arriving in Turkey on Monday amid tensions between Moscow and Ankara over the Syrian crisis, Putin reiterated that Russia is not an advocate of the Syrian government, but it will not accept a repeat of NATO's bombing of Libya.

Putin and Erdogan acknowledged but downplayed their differences over how to end the Syrian conflict. They agreed on the need to end the conflict.

Moscow is a key ally of Syria while Ankara is supporting the opposition trying to replace Assad. The two countries' relations deteriorated when Turkey forced a Syria-bound plane to land in Turkey in October, claiming that it had Russian-made defense equipment on board. Moscow said the plane was legally carrying radar parts for Syria.

AFP-Reuters-Xinhua