Washington - President Barack Obama warned Syria on Monday that deploying chemical weapons is "totally unacceptable," after what U.S. officials said were new intelligence reports that the Damascus government is preparing such munitions for possible use.

Obama told the government of President Bashar Assad that "there will be consequences, and you will be held accountable" if it used any part of its stockpile of chemical weapons, including sarin gas, the deadly nerve agent.

A U.S. intelligence official said "we have pretty good visibility" into Syria's depots, and a second U.S. official said intelligence gathered in recent days has raised alarms. The second official said it was unclear whether the Assad government planned to move beyond the preparation stage to deploying the weapons.

After months of dogged, siegelike fighting, rebel forces have begun to make significant advances in Syria, raising questions about Assad's durability and desperation.

The Obama administration has resisted any direct intervention in the conflict, but hard evidence that Syria had weaponized its chemical stocks could trigger the use of U.S. troops to secure the materials. A senior American official, who like other officials spoke on the condition of anonymity to discuss contingency plans and intelligence, said some U.S. forces have begun to run drills so they are ready to seize chemical weapons in Syria if ordered.

Syria's Foreign Ministry said the government planned no such escalation of the 20-month-old civil war. "Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the ministry said in a statement.

The White House had previously warned Syria about the use of chemical weapons, but Monday brought fresh statements from senior U.S. officials, beginning with Secretary of State Hillary Rodham Clinton during a visit to Prague.

"This is a red line for the United States," Clinton said. "I'm not going to telegraph in any specifics 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 to say we are certainly planning to take action."

White House spokesman Jay Carney said, "We are concerned that 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."

Heavy fighting continued Monday around Damascus as Assad's forces tried to strike back against rebels who have gained control of some of the suburbs around the capital.

Citing the deteriorating security situation, the United Nations announced Monday that it is withdrawing "all nonessential international staff" from the country. Radhouane Nouicer, the U.N. regional humanitarian coordinator for Syria, said that up to a quarter of the about 100 international staffers working for U.N. agencies in Syria could leave by the end of the week and that remaining staff members would be restricted to Damascus.

Syria is thought to have several hundred surface-to-surface ballistic missiles capable of carrying chemical warheads, threatening U.S. allies, including Turkey and Israel. Clinton will argue for additional NATO antimissile protection for Turkey during meetings in Brussels with alliance members Tuesday, U.S. officials said.

Clinton spoke Monday in the Czech Republic, which has been acting as the United States' diplomatic agent in Syria since the U.S. Embassy in Damascus was shuttered. Her statement echoed one made in August by Obama, who declared that Assad's deployment or use of chemical weapons would be a "red line" and would change the "calculus" for U.S. intervention in the Syrian civil war, which has killed an estimated 40,000 people.

The administration has never publicly spelled out how it would respond, but one option is an airstrike to destroy supplies before they can be weaponized. Once the chemicals were ready for deployment, however, airstrikes would no longer be viable as they could release deadly agents.

"We once again issue a very strong warning to the Assad regime that their behavior is reprehensible," Clinton said. "Their actions against their own people have been tragic. But there is no doubt that there is a line between even the horrors that they have already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."

Syria is suspected to possess the world's third-largest stockpile of chemical weapons after the United States and Russia. The chemicals are widely dispersed, with the most dangerous stocks kept in bunkers in about a half-dozen locations nationwide, U.S. officials said.

Czech Republic Foreign Minister Karel Schwarzenberg did not repeat Clinton's tough denunciations of Assad. He said chemical weapons also would pose a danger in the hands of the rebels.

"This chaotic situation of a civil war is, with the existence of these kind of arms in the country, highly dangerous," Schwarzenberg said.

NATO is expected to approve new missile defenses for Turkey at a meeting today, but a U.S. official traveling with Clinton said there are no plans to broaden that defense to include a no-fly zone or other more-assertive military measures in Syria.

The official spoke on the condition of anonymity ahead of a closed-door session of NATO foreign ministers.

"Turkey has made a request for assistance in dealing with a potential threat, and this request is in the context of defensive purposes," the official said. The deployment would not create a de facto safe zone for rebel fighters because it applies only to Turkish airspace, the official said.

Russian President Vladimir Putin, who met with Turkish Prime Minister Recep Tayyip Erdogan in Istanbul on Monday, said the deployment of missile defenses could "exacerbate" rather than "defuse" tensions on the Turkish-Syrian border.

Putin added: "We are not inveterate defenders of the current regime in Syria. . . . Other things worry us, like what will happen in the future?"