The Syrian regime has begun using missiles and barrel bombs against opposition rebels in the past week or so, a top US official said Wednesday, denouncing the increasing use of "vicious weapons."

Amid reports that Scud missiles have been fired, State Department spokeswoman Victoria Nuland said she "was not in a position to confirm types of missiles, simply to say that we're seeing missiles employed now."

"I would also say that we're seeing use of another egregious weapon. It's kind of a barrel bomb, which is an incendiary bomb that contains flammable materials," she told journalists.

Citing US administration officials, the New York Times reported Wednesday that the regime of President Bashar al-Assad had fired Scud missiles from the Damascus area against rebels in northern Syria recent days.

"The total number is probably north of six now," another US official told the Times.

Nuland said she could not confirm the type of missiles being used as that would reveal US intelligence, and White House spokesman Jay Carney also said he could not discuss intelligence matters.

But this marks a significant escalation of the fighting in the 21-month rebellion, which has already claimed some 42,000 lives, according to a British-based watchdog.

"The idea that the Syrian regime would launch missiles within its borders at its own people is stunning, desperate and a completely disproportionate military escalation," Carney said.

The report came just after US President Barack Obama said his administration recognized the Syrian National Coalition as the legitimate representative of the Syrian people, opening the way for increased US aid to the rebels.

"As the regime becomes more and more desperate, we see it resorting to increased lethality and more vicious weapons moving forward," Nuland said.

Such weapons were "very, very concerning and indicative of the regime's desperation and the regime's brutality."

She said the barrel bombs were "incendiary bombs which contain flammable material that can be like napalm" or can be packed with nails and launched from the air or from a launcher.