The United Nations Humanitarian Chief says he is concerned about the deteriorating humanitarian situation in northern Sri Lanka, where thousands of civilians are trapped in a rebel-controlled area, unable to flee combat between the fighters and the government.

John Holmes made a three-day visit to Sri Lanka last week as a sign of the international community's growing concern over the humanitarian situation in the island-nation's conflict zone.

There, the government says it is finally about to crush the separatist Liberation Tigers of Tamil Eelam, or LTTE, after 25 years of civil war.

In a closed-door session Friday, Holmes briefed the 15-member Security Council on his trip. Afterwards he told reporters the United Nations top priority is helping the thousands of civilians, who along with the rebels, have been pushed by the government into a small, narrow area along the coast.

"There are extreme concerns about the safety of civilians in that pocket," he said. "Concerns about civilian casualties, concerns about humanitarian access to them and humanitarian relief supplies for them. That is the top concern we have, which is one of the main reasons I went there myself last week, to talk to the government about that. To try to make sure they were doing everything they could to protect the civilians there."

Holmes said there is strong evidence that the LTTE are preventing the civilians from leaving. He said the estimate of the number of civilians caught in the violence varies from 70,000 according to the government, to around 200,000 by the United Nation's count, to as high as 300,000 or more according to Tamil groups.

Holmes said displaced persons were in generally good physical condition, but there is serious overcrowding in some of the transit centers. He said U.N. agencies and aid groups have relatively free access to these sites, but that movement into and out of them is very restricted - which he said is unacceptable.

He said the Sri Lankan government expressed its commitment to helping the displaced return home, promising some 80 percent would be home by the end of this year.

Regarding reports that the government has stopped using heavy weapons in an effort to spare civilian casualties, Holmes said he could not confirm that.

"I am afraid we cannot verify that. They assured me at every level that they had virtually stopped using heavy weapons, and that indeed, they were taking more casualties as a result of that [heavy weapons]," he said. "But I cannot verify that. It is simply unclear how far that is true."

But he said the U.N. believes dozens of people are being killed and many more wounded each day.

The U.N. humanitarian chief said he also urged the Sri Lankan government to refrain from any final military battle in order to give civilians time to get out safely.

Tamil Tiger rebels have been fighting the government since 1983 to establish an independent homeland for Sri Lanka's ethnic minority Tamils, who they say face second-class treatment by the Sinhalese majority. More than 70,000 people have died in the conflict.