Mounting violence between rebel fighters and government troops along northern Sri Lanka has prompted U.N. Secretary-General Ban Ki-moon to call for the safety of civilians as humanitarian groups try to provide aid to some 250,000 people trapped in the region.

“The secretary-general is deeply concerned about the safety and well-being of civilians caught in intensified fighting in the Vanni region of Sri Lanka,” a spokesman for Ban said in a statement Monday.

Ban called on the government and the separatist Tamil Tigers to respect “no-fire zones” and civilians areas, including schools, hospitals and humanitarian posts. He also asked both sides to allow the 250,000 civilians trapped in the fighting to move to “safe areas.”

Sri Lankan soldiers seized a key rebel stronghold in a surprise attack Sunday, even as humanitarian agencies feared for the safety of civilians. Video Watch a report on the recent fighting »

“It’s an incredibly serious situation,” James Elder, a U.N. spokesman, said Monday. “We have a very large number of people, including tens of thousands of children, trapped in a fast-shrinking conflict zone.”

Troops crossed a lagoon and entered the town of Mullaittivu before encountering heavy resistance from Tamil fighters, according to the government-run news agency.

“Our troops fought their way through a 40 km (25 mile) thick jungle track,” Lt. Gen. Sarath Fonseka said in a televised address Sunday.

“This is the long-awaited victory and I am happy to say that our heroic forces today captured the Mullaittivu town after 12 years,” the Sri Lanka Army chief said.

There has been no confirmation from the rebels that the strategic garrison has been overtaken.

The Liberation Tigers of Tamil Eelam (LTTE) — commonly known as the Tamil Tigers — have fought for an independent homeland for the country’s ethnic Tamil minority since 1983. The civil war has left more than 70,000 people dead.

The rebels gained control over Mullaittivu in 1996 and established a military garrison there, according to the government.

In recent days, the military has said it has made significant progress in its campaign to recapture rebel strongholds.

Earlier this month, troops regained control of the northern town of Elephant Pass, the point at which mainland Sri Lanka links to the northern Jaffna peninsula.

It had been in rebel hands for more than nine years.

The recapture enabled the government to use a highway linking the mainland to the peninsula to move troops and supplies. Previously, it was done by air and sea.

“The area that the LTTE has dominated has shrank phenomenally,” Sri Lankan High Commissioner to India, C.R Jayasinghe, told CNN. “They lost … about 90 percent of what they had.”

U.N. demands safety for Sri Lankan civilians – CNN.com