Forces loyal to Syrian President Bashar Assad have fired Scud missiles at rebels trying to overthrow Syria's government, a senior U.S. official said on Wednesday.

Use of the missiles was seen as escalation in Assad's attempt to retain power.

A U.S. official, speaking on condition of anonymity, confirmed the use of Scuds.

In Brussels, a NATO official also said on Wednesday that a number of short-range ballistic missiles have been launched inside Syria this week.

"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," the NATO official said.

Speaking separately, U.S. State Department spokeswoman Victoria Nuland, while not confirming the use of Scuds, said the United States has seen the use of incendiary barrel bombs in recent days.

"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.

On Tuesday, President Barack Obama granted U.S. recognition to a Syrian opposition coalition as the legitimate representative of the Syrian people, a move aimed at ratcheting up pressure on Syrian President Bashar Assad to leave power.

"Recognition of Syrian opposition group meant to boost those working toward political transition, democratic future," Nuland said. The head of the coalition, Moaz al-Khatib, Nuland added, was invited to Washington "as soon as it's convenient to them."