Up to 50 Pakistani troops have gone missing amid heavy fighting with pro-Taleban militants near the Afghan border, the army says.
The Pakistani army is under increased pressure in Waziristan
It said that the soldiers disappeared after militants attacked security posts and an army patrol near the town of Mir Ali in the region of North Waziristan.
The army said that about 60 suspected militants and 20 soldiers died in clashes in the area over the weekend.
Meanwhile Gen Ashfaq Kayani has become the new vice-chief of the army.
The former head of Pakistani intelligence is expected to become overall head of the army shortly if President Pervez Musharraf resigns from this position as promised.
He would inherit an army under growing pressure from insurgents in the tribal areas near the Afghan border.
"We have reports of around 50 troops missing. They are out of communications and their whereabouts have not been found," chief military spokesman Maj Gen Waheed Arshad told the AFP news agency.
The army says that the troops have been missing since Monday morning following clashes near Mir Ali, the second biggest town in the tribal district of North Waziristan.
This follows a weekend of heavy fighting. After a number of attacks on military convoys, the army sent helicopter gunships and jet fighters to target suspected militant positions in several villages.
Scores of people were killed. The army says they were militants and soldiers. But locals report at least 10 civilians were among the dead.
Correspondents say that these military campaigns are deeply unpopular in Pakistan.
They are widely seen as carried out under American pressure since Washington wants Pakistan to crack down on pro-Taleban and al-Qaeda fighters near the Afghan border.
It is believed the upper ranks of the army broadly accept this policy whatever their personal misgivings. That includes the new deputy head of the army, Gen Kayani.
Gen Kayani faces a huge challenge in Waziristan
Analysts say the danger is in the lower ranks where morale seems to be low.
Another batch of more than 200 soldiers were captured recently by militants, and appeared to have surrendered without a fight.
Correspondents say that Gen Kayani will need all his skills to tackle the growing insurgency in the tribal areas.
Pro-Taleban militants have stepped up attacks on the army after a ceasefire collapsed in mid-July.