Pakistani troops backed by jet fighters and artillery killed about 50 militants in a volatile northwestern tribal region near Afghanistan where the country's top Taliban leader is believed to be entrenched with thousands of his fighters, officials said.

They were the first-known militant casualties in South Waziristan – where Pakistan Taliban head Baitullah Mehsud and al-Qaeda figures are believed to be hiding – since the military started pounding the area with artillery about a week ago. Mehsud is blamed for a series of suicide attacks which have killed more than 100 people since late May.

Although the army has not announced a formal start of full-scale operations in South Waziristan ? an offensive which the US has been pressing Pakistan to undertake ? officials said troops are already occupying strategic positions in the region.

The operation, seen as a test of nuclear-armed Pakistan's resolve against an insurgency that has expanded in the past two years, could be a turning point in its sometimes half-hearted fight against militancy. It could also help the war effort in Afghanistan, because the tribal belt has long harboured militants who launch cross-border attacks.

Jet fighters flattened two abandoned militant-linked seminaries and a training facility yesterday in a clear sign that the operation was ramping up.

Two intelligence and army officials said heavy fighting was under way in the villages of Barwand and Madijan, with about 50 militants killed.

There was no immediate comment from the military, and the reports could not be independently confirmed due to restrictions on media access to the region.