"And we are concerned that in an increasingly beleaguered regime, having found its escalation of violence through conventional means inadequate, might be considering the use of chemical weapons against the Syrian people," White House spokesman Jay Carney said during a media briefing. "And as the president has said, any use or proliferation of chemical weapons by the Syrian regime would cross a red line for the United States."
"The Assad regime must know that the world is watching, and that they will be held accountable by the United States and the international community if they use chemical weapons, or fail to meet their obligations to secure them," Carney said.
Meanwhile, U.N. agencies in Syria are curtailing staff movements and moving facilities because the country's capital is being affected more and more by unstable conditions, officials said.
"The security situation has become extremely difficult, including in Damascus," Radhouane Nouicer, the U.N. regional humanitarian coordinator in Syria, told Integrated Regional Information Networks Monday. "For as long as international humanitarian law is not fully observed by all parties to this conflict and for as long as the safety of humanitarian workers is not strictly secured, U.N. agencies have to review the size of the their presence in the country as well as the way they deliver humanitarian aid."
Eight U.N. employees have been killed since the conflict began in March 2011 between anti-government protesters and President Bashir Assad. Eighteen volunteers of the Syrian Arab Red Crescent also have died.
Earlier Monday, Secretary of State Hillary Clinton said the United States would "take action" if Syria used chemical weapons against its people.
"I am not going to telegraph in any specifics [about] what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people. But suffice it to say we are certainly planning to take action if that eventuality were to occur."
In response, the Syrian Foreign Ministry said the government "would not use chemical weapons, if it had them, against its own people under any circumstances," The New York Times reported. The response was carried by Syria's state broadcaster.
On the fighting front, witnesses told CNN an unknown number of people were killed or wounded Monday when Syrian warplanes bombed Ras al-Ain, a town near the country's border with Turkey.
One witness told CNN fearful civilians, many "with arms and legs missing," were fleeing to the border where they were being picked up by ambulances.
Clinton commented on Syria after meeting in Prague with Czech Republic Foreign Minister Karel Schwarzenberg, indicating the two diplomats had discussed Syria, including a potential chemical weapons threat.
Schwarzenberg, characterizing the situation in Syria as "rather chaotic" and "highly dangerous," said Czech troops specializing in detecting chemical weapons and decontamination were in Jordan training forces.
Clinton was en route to a two-day meeting of NATO foreign ministers in Brussels to discuss the situation in Syria.
Turkish officials told Britain's Guardian newspaper they had credible evidence if the Syrian government's aerial bombardment against opposition-held areas failed to subdue the rebels, the Assad regime would consider using missiles topped with chemical warheads in a desperate last effort to survive.
In Washington, Carney said: "We believe they [chemical warheads] remain in the possession of the Syrian regime, the Assad regime. But, as the regime has lost all legitimacy to lead Syria, and the opposition grows in strength, our concern about the regime's intentions regarding its chemical weapons stockpiles [have] increased."
Carney said officials were monitoring the situation in Syria closely as well as the regime's chemical weapons stockpiles, but declined to say what would happen nor discuss intelligence matters.
The Syrian Network for Human Rights said 3,486 people were killed in November in the fighting between government forces and rebels seeking Assad's ouster.
"In other words, 117 are killed a day, or 5 killed every hour," the network said in its report.
"We note that there are many cases where the Syrian Network for Human rights was not able to document deaths. The cases include massacres, sieges and cut-off communication," the organization said. "Thus, the death toll is possibly much higher than what was documented."
The network said it holds Assad responsible for the killings, making the Iranian government and other Assad backers accountable, too.