By Barbara Starr, Jill Dougherty and Mike Mount
American military satellites picked up and confirmed the infrared signature of four short-range Scud missiles launched from the Damascus area to northern Syria in the past several days, a U.S. official said.
The missiles did not land on the Turkish side of the border but "came close," he said.
The Defense Support Program satellites are programmed to pick up the infrared signature of a man made or natural event.
The official could not be identified because of the sensitive nature of the information.
While not confirming specifics, State Department spokeswoman Victoria Nuland said the Syrian regime is using more lethal weapons.
"As the regime becomes more and more desperate, we see it resorting to increased lethality and more vicious weapons moving forward and we have in recent days seen missiles deployed," Nuland said.
Earlier this month, a U.S. official said the Syrian regime was "ratcheting things up," noting its forces fired nearly 20 rockets with a relatively long range of 60 miles, which it had not done before.
The use of Scuds would be a further escalation of the offensive posture. Experts said Syrian President Bashar al-Assad has up to 400 short and medium-range Russian-developed missiles.
As the Syrian military uses weapons with increased range and lethality, NATO is preparing to send a Patriot missile defense system to neighboring Turkey. Turkey made the request, which was agreed to last week.
Within a number of days, the Obama administration is expected to issue orders determining the number of Patriot missile batteries and personnel to be sent by the United States, according to a Pentagon spokesman.
Defense Secretary Leon Panetta and NATO have endorsed the plan.
The escalation involving Scuds comes as the United States said another concern, indications the Syrian military was preparing chemical weapons for possible use, had ebbed some since the intelligence was revealed and various world leaders warned Syria against their use.
Panetta told CNN's Erin Burnett that the United States is closely watching what the Syrian government is doing with its stockpiles of chemical and biological weapons (CBW).
"We've been in very close collaboration with adjoining countries - Turkey, Jordan, Israel - all very concerned that CBW doesn't fall into the wrong hands. ... We are monitoring it as best we can," Panetta said in an interview to be aired Wednesday night on CNN's "Erin Burnett OutFront."
"I can't say that this is 100%, but we usually have pretty good intelligence about some of the things that are happening," Panetta said.
He said the United States would know if Syria decides to use its chemical stockpiles.
Separately, the State Department said al-Assad's forces started using so-called "barrel bombs," an incendiary explosive with flammable material.
"It's sort of a napalm-like thing and it's completely indiscriminate in terms of civilians," Nuland said.
Human Rights Watch said such weapons produce extremely painful burns, often down to the bone, which are very hard to treat. The group said there have been four locations where incendiary weapons have been used since mid-November.