Nato is set to approve the deployment of Patriot missile interceptors to defend Turkey's border with Syria.
A meeting of the 28-member alliance's foreign ministers in Brussels follows a request from Turkey to boost its defences along the border.
Nato officials have made clear such a move would be purely defensive.
Earlier, US President Barack Obama warned Syrian President Bashar al-Assad he would face "consequences" if he uses chemical weapons against his people.
"The world is watching. The use of chemical weapons is and would be totally unacceptable," said Mr Obama in a speech at the National Defense University in Washington.
"If you make the tragic mistake of using these weapons there will be consequences and you will be held accountable."
A Syrian official has insisted it would "never, under any circumstances" use such weapons, "if such weapons exist". Rebel gains
A Nato team has already visited a number of sites in Turkey in preparation for the deployment of Patriot batteries, which could be used to shoot down any Syrian missiles or warplanes that stray over the border, says BBC defence correspondent Jonathan Beale.
The missile deployment is likely to be approved, adds our correspondent, despite opposition from Russia, whose foreign minister is also attending Tuesday's meeting in Brussels.
But analysts say any deployment - possibly supplied by the US, Germany or the Netherlands - could take weeks.
Syrian opposition fighters have reportedly made dramatic gains recently, and several government mortar shells - aimed at rebel targets close to the border - have landed across its 900-km (560-mile) border with Turkey.
Ankara's request for Nato to deploy the anti-missile batteries came after intelligence assessments that Damascus was contemplating using ballistic missiles, potentially armed with chemical warheads, reports say.
Syria is believed to hold chemical weapons - including mustard gas and sarin, a highly toxic nerve agent - at dozens of sites around the country.
The CIA has said those weapons "can be delivered by aircraft, ballistic missile, and artillery rockets".
One unnamed US official told the New York Times on Monday of "potential chemical weapon preparation". The White House says the level of concern was such that Washington is preparing contingency plans.
Russian President Vladimir Putin has warned against the missile deployment, fearing it would "exacerbate" rather than "defuse" tensions along the border.
Moscow has remained a key ally of Syria during the 22-month conflict, in which activists say more than 40,000 people have died. Ankara now backs the rebels trying to oust Mr Assad.
After talks with Turkish Prime Minister Recep Tayyip Erdogan in Istanbul on Monday, Mr Putin said the two countries had reached no breakthrough on how to "regulate the situation" in Syria.
But he emphasised that Moscow's backing for Damascus was not the same thing as supporting Mr Assad's regime.
"We are not protecting the Syrian government, we are not its advocate," he said.
On Monday the United Nations said it was pulling "all non-essential international staff" out of Syria, with as many as 25 out of 100 international staff expected to leave this week.
Although the head of the Arab League Nabil al-Arabi told AFP on Monday that the Syrian government could fall "any time", it still holds the capital, parts of the second city Aleppo, and other centres.
Syrian Foreign Ministry spokesman Jihad Makdissi is said to have fled the country, amid reports he has been dismissed, ostensibly for making statements out of line with government policy.