Colombo: Human Rights Watch on Saturday accused Sri Lankan forces of repeatedly striking hospitals in the northern war zone with indiscriminate artillery and aerial attacks that have killed scores of people, a charge the military denied.

The New York-based group claimed military commanders responsible for ordering or conducting such attacks "may be prosecuted for war crimes."

The accusation came amid growing international concern for an estimated 50,000 civilians caught in the fighting between government forces and the separatist Tamil Tigers.

In the latest heavy clashes, troops pushed further into the rebel territory on Friday, capturing an earthen fortification. A military statement said soldiers found 35 bodies of rebel fighters killed in the fighting.

There was no comment from rebels and it is difficult to verify the military's claims because reporters and independent observers are barred from the war zone.

Government troops in recent months have ousted the rebels from their strongholds across the north and say they have cornered the Tigers in a sliver of land just four kilometres-long on the northeast coast.

Human Rights Watch said on Saturday it has information "about at least 30 attacks on permanent and makeshift hospitals in the combat area since December 2008" and blamed government forces.

"Hospitals are supposed to be sanctuaries from shelling, not targets," Brad Adams, the group's Asia director, said in a statement.

One of deadliest attacks took place on May 2, when artillery shells struck Mullaivaikal hospital killing 68 people wounding 87, the statement said.