The reported attacks come despite the presence of the U.N. observers, who have reported cases of cease-fire violations from the government and the opposition.

The cease-fire went into effect April 12 and is part of a six-point peace plan negotiated by U.N. and Arab League joint special envoy Kofi Annan.

Annan's plan includes allowing humanitarian groups access to the population, releasing detainees, starting a political dialogue and withdrawing troops from city centers -- a mandate the government has not met, according to the United Nations.

The U.N. observer mission is tasked to monitor the cease-fire and the peace plan.

Maj. Gen. Robert Mood, head of the mission, said 50 observers were in the country, "deployed in the provinces of Damascus, Homs, Aleppo, Hama, Idleb and Daraa," the state-run Syrian Arab News Agency said.

A U.N. peacekeeping official in New York said that as of noon ET Thursday, 33 military observers and 27 international civilian staff were in Syria.

A total of 300 are expected to be in the country by the end of the month.

A U.N. official said this week said both sides have violated the cease-fire.

Though the United States seeks an end to all violence, most of the attacks have been by the government forces, said Mark Toner, the U.S. State Department spokesman.

"So far, the Syrian regime has taken, really, almost no steps toward fulfilling the core commitments of the Annan proposal," he said.

Syria's protests started peacefully in March last year, but a government crackdown spawned violence that has left thousands dead and prompted some military defectors to take up arms against the regime forces. The government has consistently blamed the violence on "armed terrorists."

The United Nations estimates that at least 9,000 people have died in the conflict while opposition groups put the death toll at more than 11,000.

President Bashar al-Assad's family has ruled Syria for 42 years.