COLOMBO, Sri Lanka - The Sri Lankan military and the Tamil rebels may have committed war crimes, the UN human rights chief said Friday, warning that civilian casualties could reach "catastrophic" proportions if the two sides do not suspend their fighting.

The endangered civilians are trapped in a shrinking rebel enclave in the island's war-ravaged north, which government troops are battling to capture and end a 25-year civil war.

"We need to know more about what is going on, but we know enough to be sure that the situation is absolutely desperate. The world today is ever sensitive about such acts that could amount to war crimes and crimes against humanity," UN High Commissioner for Human Rights Navi Pillay said in a statement.

Meanwhile, Secretary of State Hillary Rodham Clinton telephoned President Mahinda Rajapaksa on Friday to discuss civilian casualties and ways to help civilians trapped in the war zone, a government statement said.

The government continues to send essential supplies for civilians in and around the conflict zone, the statement said.

However, Pillay said the army has repeatedly shelled inside safe "no-fire" zones set up for the civilians, and that "a range of credible sources" showed that more than 2,800 civilians had been killed and more than 7,000 wounded since January 20.

Both figures are higher than previous estimates, and Pillay said 150,000 to 180,000 remained trapped in the rebel area -- estimated at 35 square kilometres -- on Sri Lanka's northeast coast.

"The current level of civilian casualties is truly shocking, and there are legitimate fears that the loss of life may reach catastrophic levels, if the fighting continues in this way," said Pillay, a South African who is a former UN war crimes judge.

The government denied the allegations, and accused the Tiger rebels of using the civilians as a human shield in a desperate attempt to avoid being defeated.

"We have very clearly stated that we have not at any time fired at the no-fire zone," said Mahinda Samarasinghe, the minister for disaster management and human rights. Samarasinghe said the UN report used "unsubstantiated figures."

He said that the government has decided not to used its heavy guns to protect the civilians even though the LTTE had been doing so.

Pillay's statement said the Tamil Tigers, fighting since 1983 to establish a separate state in the north of Sri Lanka, may have been using the civilians as human shields and shooting at people trying to flee. Their alleged actions "should be examined to see if it constitutes war crimes," Pillay said.

The government has rejected calls from international aid groups for a cease-fire, saying it is on the verge of victory.

Rebel officials could not be reached. Most communication to the north has been severed, and accounts of the fighting could not be verified independently because independent journalists are barred from the war zone.

Pillay urged the government to grant full access to UN and other agencies to allow an accurate assessment of the human rights and humanitarian conditions in the conflict zone.

The Liberation Tigers of Tamil Eelam have fought since 1983 for an independent state for the Tamil minority, which suffered decades of marginalization at the hands of governments dominated by the Sinhalese majority. More than 70,000 people have been killed in the fighting.