Indian officials took a demand for an immediate cease-fire in Sri Lanka's civil war to the country's president on Friday as the U.N. reported that nearly 6,500 ethnic Tamil civilians were killed in the last three months.

Concern for the safety of civilians trapped in the ever-shrinking war zone has increased in recent weeks as the government pushed ahead with its offensive to crush the rebels and end the nation's quarter-century civil war.

The U.N. estimates that 50,000 people are still trapped in the war zone, spokeswoman Elisabeth Byrs said Friday.

Brig. Shavendra Silva, a top commander in the conflict zone, said intelligence reports indicated reclusive rebel leader Velupillai Prabhakaran and other top Tamil Tiger officials remain holed up in the sliver of territory still under the guerrillas' control.

On Monday, the military broke through rebel fortifications on the edge of a previously declared "no-fire" zone along the northeastern coast, sparking an exodus of more than 100,000 civilians. The rebels said at least 1,000 civilians were killed in that battle and the Red Cross said hundreds had been killed or wounded.

Neighboring India, under pressure from its own Tamil population in the midst of a national election, sent National Security Adviser M. K. Narayanan and Foreign Secretary Shivshankar Menon to Sri Lanka on Friday to push for a cease-fire. The officials met with President Mahinda Rajapaksa, but details of the meeting were not immediately available.

"We are very unhappy at the continued killing in Sri Lanka. All killing must stop. There must be an immediate cessation of all hostilities," External Affairs Ministry spokesman Vishu Prakash said in a statement Thursday.

The military said it was pushing ahead with its offensive, engaging the rebels in heavy fighting Thursday in the tiny coastal strip still held by the Tamil Tigers, who once controlled a vast area of northern Sri Lanka.

Doctors Without Borders, a medial relief group, said the civilians pouring out of the conflict zone included large numbers of people who have sustained blast, mine and gunshot wounds.

International rights groups have accused the government of shelling densely populated civilian areas in the war zone and accused the rebels of holding the civilians as human shields. Both sides deny the accusations.

At least 6,432 civilians were killed in the intense fighting over the past three months and 13,946 wounded, according to a private U.N. document circulated among diplomatic missions in Sri Lanka in recent days. The casualties were reported as "verified data" in the document, which was given to The Associated Press by a foreign diplomat Friday.

The U.N. has declined to publicly release its casualty figures and had no immediate comment on the document.

The level of civilian deaths has increased dramatically as the fighting has worn on, according to the U.N. An average of 33 civilians were killed each day at the end of January, a number that jumped to 116 by April, the document reported. More than 5,500 of those killed were inside a government-declared "no-fire" zone, the report said.

Foreign Secretary Palitha Kohona said the government took special care to avoid civilian casualties, and said many of those killed were combatants dressed in civilian clothing.

U.N. Secretary-General Ban Ki-moon said he would send humanitarian experts to Sri Lanka to monitor the situation. The government agreed in principle to accept a team but the details needed to be worked out, said Human Rights Minister Mahinda Samarasinghe.

More than 106,000 civilians have fled the fighting since Monday, according to the government. The U.N. refugee agency gave similar figures on Friday.

Aid workers and diplomats said the mass exodus of civilians was overwhelming government facilities in the region. The reports cannot be independently verified because journalists are barred from the war zone and the camps for those displaced by the fighting.

"We're very concerned that the humanitarian provisions in place to receive these people are not sufficient to meet immediate needs," said U.N. spokesman Gordon Weiss.

Meanwhile, a Sri Lankan court released an ethnic Tamil newspaper editor Friday after holding him for nearly two months on accusations he aided a rebel air raid near the capital, Colombo, said his attorney K.V. Thavarasha.

Police investigations into allegedly suspicious telephone conversations during the attack that killed four yielded no evidence against the journalist, Thavarasha said.

Sri Lanka has come under heavy international criticism after a spate of attacks and arrests of journalists viewed as critical of the government's offensive against the rebels, who have been fighting for 25 years to create an independent homeland for ethnic minority Tamils.