COLOMBO, Sri Lanka - More than 150,000 civilians are trapped in a rapidly shrinking jungle battlefield as Sri Lanka hopes to crush a 25-year-old separatist movement.

The top U.N. official in the country said Monday their lives are in serious danger.

"There's so many people, so many guns and such a high intensity of fighting," U.N. resident coordinator Neil Buhne told The Associated Press. "There have been many civilians killed over the last two days. ... It's really a crisis now."

The huge population of trapped, uprooted civilians is living in makeshift shelters under desperate conditions in the last scrap of rebel-controlled territory, Buhne said. They have been without U.N. food aid since Jan. 16, when aid convoys were no longer allowed.

The civilians are in the crossfire between tens of thousands of advancing government troops fresh from capturing the Tamil Tigers' last stronghold over the weekend, and an unknown number of rebels who are so committed to their cause that some wear suicide vials of cyanide around their necks in case of capture.

The Tamil Tigers have fought since 1983 to create a separate state for minority Tamils, who have suffered decades of marginalization at the hands of governments controlled by the Sinhalese majority. More than 70,000 people have been killed in the civil war.

Human rights groups and diplomats have expressed growing concerns about the safety of the trapped civilians, whose numbers are estimated at 150,000 to nearly 400,000. The government says the number is far lower.

Many have accused the rebels of preventing the civilians from fleeing the area, while the government has accused the insurgents of using them as human shields.

Rebel officials could not be reached for comment because communications to the war zone, in the northern part of the country, have been cut.

The war zone is closed to most independent observers, including foreign journalists, and it is unclear how many civilians are trapped there.

No 'safe zone' in jungle

But with the desperate rebels and the civilians squeezed into a 115-square-mile area in the jungle roughly the size of Philadelphia, the situation has grown far more dangerous.

"It's a recipe for trouble ... unless there's maximum restraint on both sides," Buhne said, adding civilians should be allowed to leave the war zone.

He said until recently, both sides in the fighting worked hard to avoid civilian casualties.

Civilians are suffering from poor nutrition, no sanitation facilities and an inadequate water supply, he said. Some have been displaced as many as 15 times by the fighting, and many have dug trenches to take shelter during the shelling.

The government last week unilaterally declared a "safe zone" in a small section of rebel-held territory and called on all civilians to move into that area, where they would be protected.

But there have been repeated reports of artillery fire in that region, and Buhne said there was regular fighting and shelling in the "safe zone" in recent days.

"It's hard for us to assign responsibility, but there's definitely been fighting," he said.

The military has repeatedly denied fighting in that area.

"We are not targeting this safe zone," military spokesman Brig. Udaya Nanayakkara said. "(The rebels) have moved their weapons to the safe zone and are firing from the safe zone, but we don't engage them."

Meanwhile, troops on Monday consolidated their control of the Tamil Tigers' last stronghold, Mullaittivu, while other forces fought pitched battles with the rebels in the jungles in the Vishwamadu area, Nanayakkara said.

"They are engaging with artillery. They are engaging with mortars. They are engaging with small arms," he said.

One diplomat said the use of heavy artillery in the confined, densely populated area could lead to more civilian deaths than if the government relied solely on ground troops. The diplomat spoke on condition of anonymity for fear of antagonizing the Sri Lankan government.