* U.S. officials unaware of any previous use of the missiles
* Use seen as escalation in Assad's struggle to retain power
WASHINGTON/BRUSSELS, Dec 12 (Reuters) - Forces loyal to Syrian President Bashar al-Assad have fired Scud missiles at rebels trying to overthrow Syria's government, a senior U.S. official said on Wednesday, a step seen as an escalation in Assad's struggle to retain power.
U.S. officials said they were unaware of any previous instances in which Scuds were used against the rebels since the start of the 20-month-old uprising, which has killed more than 40,000 people.
White House spokesman Jay Carney declined to confirm the reports, saying he was aware of them but could not discuss intelligence matters.
"If true, this would be the latest desperate act from a regime that has shown utter disregard for innocent life," he said. "The idea that the Syrian regime would launch missiles in its borders at its own people is stunning, desperate, a completely disproportionate military escalation."
A senior U.S. official, speaking on condition of anonymity, said Scuds had been used.
In Brussels, a NATO official, who also spoke on condition of anonymity, said a number of "Scud-type" short-range ballistic missiles had been launched inside Syria in recent days.
"Allied intelligence, surveillance and reconnaissance assets have detected the launch of a number of unguided, short-range ballistic missiles inside Syria this week," the official said.
"Trajectory and distance travelled indicate they were Scud-type missiles," the NATO official said.
NATO agreed last week to send Patriot anti-missile systems to alliance member Turkey to reinforce its air defences and calm its fears of coming under missile attack, possibly with chemical weapons, from neighbouring Syria.
The NATO official said the Syrian missiles had landed inside Syria and no missiles had hit Turkish territory. He said the Western alliance had no information about what casualties or damage the missiles had caused.
Asked if there was any evidence of Syrian use of chemical weapons, he said: "We have no information concerning the payload."
U.S. President Barack Obama warned Assad last week not to use chemical weapons against Syrian opposition forces, saying there would be unspecified consequences if he did so.
The United States, Germany and Netherlands have all agreed to send Patriot missiles to protect Turkey, but the missile batteries are not expected to arrive for several more weeks.
The New York Times, which initially reported Syria's use of the missiles, quoted one official as saying more than six had been fired at the rebels. Another official said the missiles had been launched from the Damascus area at targets in northern Syria, the Times said.
U.S. officials, speaking on condition of anonymity, told Reuters they would not dispute the Times report.
Speaking separately, U.S. State Department spokeswoman Victoria Nuland, while not confirming the use of Scuds, said the United States has also been seeing the use of incendiary barrel bombs, which she said was "another egregious weapon" that was "completely indiscriminate in terms of civilians."
"As the regime becomes more and more desperate, we see it resorting to increased lethality and more vicious weapons moving forward," she told a State Department briefing.
News of Syria's use of Scud missiles broke as Western and Arab nations sympathetic to the uprising against Assad gave full political recognition to the opposition.