Separatist Tamil Tiger rebels and government forces traded artillery and mortar fire as the insurgents tried to break through government lines and recapture the northern Jaffna Peninsula, the military said Sunday.

Aid agencies, meanwhile, warned of a growing humanitarian crisis in Jaffna, where food and water is running out, access routes have been cut, and civilians are fearing for their lives.

"Consolidation operations are ongoing in Jaffna and Muhammalai," the de facto border between government and rebel-held territory in the north, military spokesman Maj. Upali Rajapakse said. "They are trying to penetrate our defense lines, we are not allowing them to do so."

CountryWatch: Sri Lanka

Rajapakse said the Tigers were trying to recapture Jaffna town and the peninsula, a Tamil-majority area they lost control of in 1995, but which they see as an integral part of an independent homeland for the country's 3.2 million ethnic Tamils.

The two sides have been heavily shelling each other's positions since Aug. 11, when the rebels made a major push to retake the peninsula, leaving hundreds of troops, rebels and civilians dead.

Rajapakse did not say how many security forces or rebels were killed in the latest exchange of fire, but said the military had lost 131 soldiers in the battle for Jaffna up until Saturday.

Another 800 rebels were killed, including 80 in two days of intermittent artillery fire in Muhammalai to Saturday, according the military.

Tiger officials were not immediately available to comment, but the two sides routinely inflate each other's death tolls.

Also in Jaffna, hundreds of soldiers raided a university, seizing computers and detaining a student leader the pro-rebel TamilNet Web site said was a third year art major. Rajapakse said the student group was a front for Tamil Tiger rebel activities, and that soldiers found rebel literature, 15 anti-personnel mines and other equipment used to build anti-tank mines on the campus.

It was not immediately possible to reach the university for comment because the military jams phone lines to prevent rebels reporting on troop movement.

The Tigers, listed in several countries, including the U.S., as a terrorist group, have been battling the government for over 20 years for a separate homeland in the north and east, where they already control wide swaths of territory.

A Norwegian-brokered cease-fire in 2002 was supposed to end the violence, in which as many as 65,000 people were killed, many of them civilians, and more than 1 million people displaced.

But violence surged in December, and both sides have since launched several major offensives in the north and east killing at least 1,500 people, according to international monitors.

In Colombo, where people are still jittery after a bomb blast last week killed seven people, police and armed troops carried out random checks on vehicles and buses entering the city.

The U.N., meanwhile, said only minimal aid has been able to reach tens of thousands of people who have been displaced by weeks of fighting in the north and east.

About 41,000 people have fled their homes in Jaffna and escaped into neighboring rebel-held territory, the U.N.'s refugee agency said Friday.

Those who cannot get out of Jaffna are seeking refuge in crowded welfare centers or schools. One 23-year-old woman, who told The Associated Press she was too scared to give her name, said she had fled her home in Warani after shelling killed her 14-year-old sister and five other people on Aug. 13.

"I can still remember the dead bodies scattered there," she said.

She was packed into a school with about 400 other people and food was running low, she said.

"Certainly we are deeply concerned about the water and food and sanitation problem," U.N. spokeswoman Orla Clinton said, calling for "unconditional and immediate access" for aid agencies.

On Saturday, government spokesman Keheliya Rambukwella said 3,800 tons of food and other aid would be shipped to Jaffna in the next few days.

The International Committee of the Red Cross said Sunday that the government had requested the ship fly an ICRC flag to ensure security on the route from Colombo to Jaffna.

A second ship was expected to leave from the eastern port of Trincomalee and evacuate foreign nationals, including aid workers, who have been stranded on the peninsula.