U.N. Secretary-General Ban Ki-moon said on a visit to Baghdad Thursday that he had expressed his "gravest concerns" to Syria's government over any use of chemical weapons and had written directly to al-Assad.
He warned that anyone responsible for the use of chemical weapons would face serious consequences.
Ban is to meet with the head of the Organization for the Prohibition of Chemical Weapons in Baghdad to discuss how it can work with the United Nations on the issue.
NATO foreign ministers agreed this week to a request by Turkey for Patriot missiles to be deployed along its border to bolster its air defenses against potential Syrian threats.
Errant Syrian artillery shells struck the Turkish border town of Akcakale and killed five Turkish civilians in October.
Early Thursday, the German Cabinet agreed to send Patriot missiles and up to 400 soldiers to Turkey to deter the Syrian civil war from spilling into the country. Germany's parliament will vote on the deployment next week, the foreign ministry said.
In addition to Germany, the United States and Netherlands, both of which have Patriot capabilities, have signaled they would be willing to contribute missiles.
"Any deployment will be defensive only. It will in no way support a no-fly zone or any offensive operation," the NATO statement said.
NATO's decision was made as the fears surfaced that the Assad regime might be preparing to use chemical weapons.
"The Syrian stockpiles of chemical weapons are a matter of great concern," NATO Secretary General Anders Fogh Rasmussen said.
"We know that Syria possesses missiles. We know they have chemical weapons and, of course, they also have to be included in our calculations," he said. "And this is also the reason why it is a matter of urgency to ensure effective defense and protection of our ally Turkey."
In the United States, Republican Sen. John McCain said Thursday that time may be running out.
"If true, these reports may mean that the United States and our allies are facing the prospect of an imminent use of weapons of mass destruction in Syria, and this may be the last warning we get," McCain said. "The time for talking about what to do may now be coming to a close and we may instead be left with an awful and very difficult decision."