A Sri Lankan soldier stands guard next to a tank which was captured from Tamil Tiger. (Eranga Jayawardena/Associated Press)

Two top Indian officials met with Sri Lanka's president Friday to demand an immediate ceasefire in the bloody civil war as the United Nations reported that nearly 6,500 ethnic Tamil civilians were killed in the last three months of fighting.

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

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.

Neighbouring 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 ceasefire.

Sri Lankan ethnic Tamil civilians cross to government controlled areas at Puthukudiyiruppu on Friday. (Eranga Jayawardena/Associated Press)

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.

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.

33 people killed daily: report

At least 6,432 civilians were killed in the intense fighting over the past three months and 13,946 were wounded, according to a private UN 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 UN 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 UN, 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.

UN Secretary General Ban Ki-moon said he would send a team of humanitarian experts to Sri Lanka to monitor the situation. The government agreed in principal to accept such 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 Doctors Without Borders aid group said those fleeing included large numbers of people suffering from blast, mine and gunshot wounds.

Government facilities overwhelmed, some say

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 UN spokesman Gordon Weiss.

The rebels have been fighting to create an independent homeland for ethnic minority Tamils, who have faced decades of marginalization by governments controlled by the ethnic Sinhalese majority.