BEIRUT, Lebanon - Syrian forces stormed student dormitories during an antigovernment protest at Aleppo University on Thursday, firing tear gas and bullets in an hours-long siege that killed at least four students and forced the closure of the state-run school, activists said.

U.N. truce observers toured other restive parts of the country, and residents told them of being too terrified to walk on the streets after dark as the 14-month-old uprising rages on. The United Nations estimates 9,000 people have been killed since the revolt began, and a peace plan brokered by international envoy Kofi Annan nearly a month ago has done little to stem the bloodshed.

In Washington, White House spokesman Jay Carney conceded that the plan might be doomed.

"If the regime's intransigence continues, the international community is going to have to admit defeat," he said, adding that new measures might have to be taken, including a return to the U.N. Security Council. He gave no further details.

It was not clear how long the university would remain closed following the siege, which began late Wednesday when around 1,500 students held a protest against President Bashar al-Assad's regime. Pro-regime students attacked the crowd with knives before security forces swept in, firing tear gas and then live ammunition, activists said.

"Some students ran to their rooms to take cover, but they were followed to their rooms, beaten up and arrested," student activist Thaer al-Ahmed said. "Others suffered cuts and broken bones as they tried to flee."

Raids and intermittent gunfire continued for about five hours through early Thursday, he said, adding that dozens of people were wounded, some critically, and 200 students were arrested.

The student quarters - known as the University City - comprise 20 dormitories that house more than 5,000 students next to the university campus. Students there often shout anti-Assad slogans from their rooms at night.