Both government forces and Tamil rebels may be breaking international humanitarian laws and must suspend their fighting to allow thousands of civilians to escape, the U.N.'s human rights head said Friday.

The civilians are caught in a shrinking rebel enclave in the island's war-ravaged north, which government troops are battling to capture to end a 25-year civil war with the Liberation Tigers of Tamil Eelam rebels.

"Certain actions being undertaken by the Sri Lankan military and by the LTTE may constitute violations of international human rights and humanitarian law," U.N. High Commissioner for Human Rights Navi Pillay said in a statement.

She said the army has repeatedly shelled inside safe zones set up for the civilians, and "a range of credible sources" showed that more than 2,800 civilians had been killed and more than 7,000 wounded since Jan. 20.

Both figures are higher than previous estimates, and Pillay said 150,000 to 180,000 remained trapped in the rebel area on Sri Lanka's northeastern 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 former U.N. war crimes judge.

The government denied allegations it has harmed civilians, and accused the Tamil 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.

He said Pillay should have appealed to the rebels to let the civilians go.

"We are perplexed and dumbfounded that the real issue has not been commented on," Samarasinghe said.

Pillay's statement did accuse the Tamil Tigers, who have fought since 1983 to establish a separate state in the north and east, of possible war crimes by using civilians as human shields and shooting at people trying to flee.

"The brutal and inhuman treatment of civilians by the LTTE is utterly reprehensible, and 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.

The Tamil Tigers 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.