(CNN) -- The NATO alliance weighed in on the Syrian crisis Tuesday, warning the Bashar al-Assad regime about using chemical weapons and mulling a Turkish request for Patriot missiles to defend its borders.
NATO's secretary-general echoed warnings from U.S. President Barack Obama that the Syrian government may be toying with the idea of using chemical weapons to crush the 21-month rebellion.
"The Syrian stockpiles of chemical weapons are a matter of great concern," Anders Fogh Rasmussen told reporters.
"We know that Syria possesses missiles. We know they have chemical weapons and, of course, they also have to be included in our calculations. And this is also the reason why it is a matter of urgency to ensure effective defense and protection of our ally Turkey," he said.
"Let me add to this that the possible use of chemical weapons would be completely unacceptable for the whole international community, and if anybody resorts to these terrible weapons I would expect an immediate reaction from the international community."
The Syrian Foreign Ministry denied that the country has any plans to use chemical weapons, state TV has reported. The government likewise has repeatedly stressed it will not use such weapons, if they exist, against its people under any circumstances.
But U.S. officials say "worrying signs" suggest otherwise.
Syrian forces have started combining chemicals that could be used to make deadly sarin gas for weapons to attack rebel and civilian populations, one U.S. official said.
The intelligence, obtained over the weekend, the official said, came from multiple sources. But the official declined to provide more details about how the United States learned of it. Sarin gas, the source said, could most readily be used to fill artillery shells.
Obama said on Monday that "the use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences, and you will be held accountable."
In other action, the Syrian civil war has spilled into Turkey, where errant Syrian artillery shells struck the border town of Akcakale and killed five Turkish civilians in October.
Turkey has asked NATO for Patriot missiles to bolster its air defenses against Syrian threats. NATO is expected to approve the request Tuesday.
The United States, Germany and Netherlands, which all have Patriot capabilities, have signaled they may be willing to contribute missiles should NATO approve the deployment to Turkey.
Rasmussen said he expects NATO ministers to make a decision "to enhance Turkey's air defenses" and expects those three nations to deploy the missiles. He said the "actual deployment will take place within weeks."
The secretary-general stressed that a deployment would be "purely defensive" and that NATO has "no intention to prepare offensive operations."
Violence raged Tuesday, one day after 239 people were killed across Syria, said the opposition Local Coordination Committees of Syria.
Undeterred by Syria's gruesome war of attrition, rebel fighters have scored a key gain where government forces once ruled.
The rebels have cut off a sprawling military base outside Aleppo with some 450 government soldiers trapped inside. The rebels could easily overrun the base, fighter Ali Jadlan said, but they want to give government soldiers a chance to defect.
Already, about 250 soldiers have escaped the siege, and most of them have joined the opposition. It's another indication al-Assad is losing his grip on a country he once firmly commanded.
The government has tried air-dropping food to its soldiers, often missing its targets. Opposition fighters have shot out their water supply.
While the soldiers still have stockpiles of artillery, their options are dwindling.
"They have reached a point where they think that they can't go back," said Jamal, a defected soldier. "They have reached a dead end. Slowly, they are weakening."
But the government isn't giving up.
Fierce artillery and missile shelling bombarded the opposition hotbed of Homs for the 35th straight day, dissidents said Tuesday, as government and rebel forces battled in neighborhood streets.
Both rebels and the government are reporting a deadly strike on a school in the Damascus area. The opposition Syrian Observatory for Human Rights said nine students were killed in a mortar strike. State TV said "terrorists" shelled the school with mortars and killed eight students and a teacher.
CNN's Nick Paton-Walsh, Holly Yan, Gul Tuysuz, Barbara Starr and Jill Dougherty contributed to this report.