New York - The United Nations said Friday that its top humanitarian official would visit Sri Lanka over the weekend as tens of thousands of civilians remain trapped in rebel-held areas. John Holmes, undersecretary general for humanitarian affairs, will be in Sri Lanka for three days, urging the government to facilitate a UN humanitarian mission to the conflict zone and assessing the condition of refugees living in makeshift camps.

The UN Security Council president, Mexican Ambassador Claude Heller, said the Sri Lankan government should allow the UN and Red Cross to access all sites where displaced people were being registered and provided with shelter, according to UN spokeswoman Marie Okabe.

The plight of civilians trapped in a narrow coastal strip in northern Sri Lanka worsened Friday amid shortages of food and medicine as the Army vowed to continue its offensive operations against Tamil rebels in the area.

The UN estimates that of the tens of thousands of civilians remaining in the conflict zone "at least 50,000 are in extreme peril."

No humanitarian aid has been delivered to the conflict zone since April 1, Okabe said, adding that the UN was concerned about malnutrition and other health issues, as well as overcrowding in the camps.

UN officials said those who escaped the fighting spoke of Tamil rebels using civilians as human shields.

In Washington, the White House said that President Barack Obama was "deeply concerned about the plight of innocent civilians caught up in the conflict."

"We call on both sides to stop fighting immediately and allow civilians to safely leave the combat zone," the US statement said.

"We call on both sides to strictly adhere to their obligations under international humanitarian law. We are very concerned about reports of violations and take these allegations very seriously."

The Sri Lankan government should stop shelling the "safe zone," let international aid groups and media reach civilians and refugees, the White House said. The US is "working with international partners to attempt to care for those civilians who can be reached."

"It would compound the current tragedy if the military end of the conflict only breeds further enmity and ends hopes for reconciliation and a unified Sri Lanka in the future," the White House said.

For more than 25 years, the Liberation Tigers of Tamil Eelam (LTTE) have been fighting for a separate state for the Tamil ethnic minority in the northern and eastern parts of majority-Sinhalese Sri Lanka. (dpa)