The group Human Rights Watch has accused Sri Lankan government forces of repeatedly shelling hospitals in the northern war zone during an ongoing offensive against separatist Tamil Tiger rebels.

The New York-based rights group issued a report late Friday quoting patients, medical staff, aid workers and other witnesses describing at least 30 attacks on hospitals since December.

Military spokesman Udaya Nanayakkara denied the allegation.  He told the Associated Press that government troops have stopped using heavy weaponry in their push to capture the last remaining rebel-held territory.

The Human Rights Watch report came amid a new wave of concern over the fate of thousands of civilians trapped alongside the rebels in a shrinking patch of territory surrounded by encroaching government troops.

The United Nations estimates that 50,000 people are stranded in the conflict zone.

In its report, Human Rights Watch also condemned the rebels for using civilians as human shields, but said "violations by one party to an armed conflict do not justify violations by the other."

The group said attacking hospitals is a violation of international law, and repeated artillery shelling of them is evidence of war crimes.

In the latest fighting, the Defense Ministry said Sri Lanka's navy sank two rebel boats during a sea battle off the northern coast Saturday, killing at least 14 rebels.

Another government statement said troops on Friday captured one of the rebels' earthen fortifications and found the bodies of 35 Tamil Tigers who had been killed in fighting.

Some information for this report was provided by AFP and AP.