NATO foreign ministers were set on Tuesday to approve Turkey's request for Patriot missiles to counter any threats from neighbouring Syria after the United States issued a blunt warning to Damascus against the use of chemical weapons.

Officials from the 28-member alliance meeting in Brussels have said that the US-made missiles, to be deployed along Turkey's border with Syria, would be for purely defensive purposes.

US President Barack Obama on Monday told Syrian President Bashar al-Assad not to use chemical weapons against his own people, in a new warning to Damascus as the conflict approaches the 21-month mark with more than 41,000 people killed.

"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."

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

The Syrian government, which is fighting hard to prevent the capital Damascus from falling to rebel forces, on Monday repeated a pledge that it would never resort to chemical weapons.

But 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 use the gas in a limited artillery attack on advancing rebels.

Washington worries that battlefield advances by rebels could prompt Assad to use chemical arms, or that such stocks could become insecure or find their way into the hands of groups hostile to the United States and its allies.

On the ground on Tuesday, the Syrian army shelled several areas east and southwest of Damascus after radical Islamist fighters seized a village in an eastern province bordering Iraq.

Rami Abdel Rahman, director of the Britain-based Syrian Observatory for Human Rights, said "the army is trying at all costs to keep the rebels out of Damascus.

"The rebels are pushing hard to enter into the city but they have not been able to make the advance they are hoping for," he added.

More than 41,000 people have been killed in violence across Syria since the outbreak of an anti-regime revolt in March last year, according to the Observatory, and several hundred thousand have fled into neighbouring countries including Turkey.

In the face of deteriorating security, the United Nations on Monday suspended operations in Syria and said it would pull out non-essential staff, while the European Union reduced its activities in Damascus to a minimum.

Against this backdrop, Syria dominates the two-day NATO meeting in Brussels, with a decision on Turkey's request for Patriot batteries to defend itself against any Syrian missile attack top of the agenda.

Russian President Vladimir Putin warned Monday that any deployment of Patriot 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 told a press conference with Turkish Prime Minister Recep Tayyip Erdogan after talks in Istanbul.

Moscow is a staunch ally of Damascus, routinely blocking resolutions against Assad's regime at the UN Security Council to the dismay of Washington and the West which has sought intervention to stem the bloodshed.

That stance and the latest exchanges may make a lunch meeting Tuesday of the NATO Russia Council attended by Clinton and her Russian counterpart Sergei Lavrov somewhat strained.

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 the point on Monday.

NATO "is a defence alliance and any deployment would be for defensive purposes," one diplomat said, stressing that it would in no way "support a no-fly zone" to protect the rebels from Damascus's still potent air power.

Military sources in Turkey have said NATO is considering the deployment of up to six Patriot batteries and some 300-400 foreign troops to operate them.

The Patriot, designed mainly to bring down missiles but effective also against aircraft, would likely be supplied by Germany, The Netherlands or the United States.