In an escalation of the conflict in Syria, the United States has determined that on Monday the Syrian military launched Scud missiles at rebel-held target areas in the northern part of the country.

A U.S. official tells ABC News that on Monday Syrian military units loyal to President Bashar Assad's regime launched at least five Scud missiles from the Damascus area toward rebel target areas in the north. This appears to be the first time that the Soviet-era short-to-medium-range missiles have been used against rebels in the two-year conflict.

A second official says three Scud missiles were launched early Monday at an area west of the northern city of Aleppo. An additional two missiles were fired later that afternoon.

Both officials tell ABC News there has not been any missile activity in the days since then. One said, "Any time Scuds are involved they're a concern," but that it was difficult to assess what damage the missiles may have caused on the ground. However, the official said there are no indications that the missiles were carrying chemical weapons.

At the State Department, spokeswoman Victoria Nuland told reporters, "We're seeing missiles employed now" in Syria against rebel fighters. She refused to identify them as Scud missiles because doing so might reveal how intelligence was collected.

Nuland said that in the last week the Assad regime has also begun using another weapon, a so-called "barrel bomb," which can be dropped from the air or deployed from launchers. She described them as barrels containing nails or incendiary material like napalm that are being indiscriminately launched at targets.

She called the Assad regime's use of both weapons as indicators of a "desperate" regime that is resorting "to increased lethality and more vicious weapons."

The Scud missile launches were first reported by the New York Times Wednesday afternoon.

Last week U.S. intelligence indicated that the Assad regime might be preparing to use chemical weapons against rebels who have been making territorial gains in Syria.

U.S. intelligence determined last week that the Syrian military had loaded elements of Sarin nerve gas onto aerial bombs at several airfields. U.S. officials said those bombs were not loaded onto Syrian aircraft.

The Syrian move prompted a public warning from President Obama that any use of chemical weapons by the President Bashar Assad's would lead to unspecified "consequences."

Defense Secretary Leon Panetta said Monday there had not been any new activity at the sites that were of concern. "We haven't seen anything new indicating any aggressive steps to move forward in that way," said Panetta.