WASHINGTON - Syrian government forces have fired at least a half-dozen short-range ballistic missiles at rebel groups in northern Syria over the last several days, according to U.S. officials, a potentially significant escalation of a civil war that has killed more than 40,000 people.

U.S. officials and the group Human Rights Watch also alleged Wednesday that Syrian government forces were dropping incendiary devices similar to napalm weapons on rebel fighters in populated areas.

The officials described the tactics as acts of desperation as rebels gain momentum in the nearly two-year-old fight to oust President Bashar al-Assad.

Details were scarce on the launch site of the missiles or whether they caused any casualties. Victoria Nuland, a spokeswoman for the State Department, said only that "we have . . . seen missiles deployed."

Administration officials, who spoke on condition of anonymity to discuss intelligence findings, declined to specify the types of missiles used or the exact location of the strikes. Some reports said they were Scuds, the type of missile used by Saddam Hussein during the 1991 Persian Gulf War.

A NATO official in Brussels, Belgium, told the Reuters news agency: "Allied intelligence, surveillance and reconnaissance assets have detected the launch of a number of unguided, short-range ballistic missiles inside Syria this week. Trajectory and distance traveled indicate they were Scud-type missiles."

A second senior Obama administration official said that as recently as two weeks ago, U.S. intelligence had dismissed reports of missile strikes by the Syrian government as unfounded but that "this time, they say it was surface-to-surface missiles of some sort."

The bulk of Syria's substantial missile arsenal consists of Russian- and North Korean-made Scuds, which have a range of up to 300 miles. There have been reports that Syria modified one variant to boost its reach to 435 miles.

Scuds, most infamously used by the Iraqi government in 1991, are notoriously inaccurate, although modifications since then are said to have improved their capabilities. In the worst attack of the 1991 war, 28 U.S. troops were killed when a Scud hit their barracks in Dhahran, Saudi Arabia.

U.S.-made Patriot missiles were effectively used against Scuds in the gulf war, and NATO has agreed to deploy Patriot batteries in Turkey, which shares a 550-mile border with Syria.

The use of Scuds or similar missiles could increase the risk of civilian casualties because they could easily stray into densely populated areas even if fired at rebel outposts owing to their inaccuracy.

As fighting continued in the suburbs of Damascus, the Syrian Interior Ministry said three bombs, at least one of them in a car, had collapsed walls of the ministry building and killed at least five people. The bombs have been a trademark of extremist groups that Western governments say are increasingly infiltrating the insurgency.

On Tuesday, the Obama administration designated one of the groups, the Nusra Front, as a global terrorist organization. The administration says the front is part of the al-Qaeda organization in neighboring Iraq.

The United States and other rebel supporters moved expeditiously Wednesday to promote a Syrian opposition political front.

More than 100 governments attending a Friends of Syria meeting in Marrakesh, Morocco, officially recognized the recently formed Syrian Opposition Coalition as "the legitimate representative of the Syrian people and the umbrella organization under which the Syrian opposition are gathering."

"Bashar al-Assad has lost legitimacy and should stand aside to allow a sustainable political transition," a declaration issued at the meeting said. President Obama announced U.S. recognition of the group Tuesday in an interview with ABC News.

A U.S. official who attended the Morocco meeting praised a speech there by the coalition's president, Mouaz al-Khatib, an Islamic cleric considered a moderate. The official said Khatib "made clear how much they're working for all Syrian people, no matter what community or religion."