Nearly 50 people have been killed after rising tension between local and foreign militants in north-west Pakistan erupted into fierce fighting.
The Waziristan area is a militant stronghold
Heavy shelling has raged since Monday near Wana in the South Waziristan tribal area close to Afghanistan.
Most of those killed were militants from Uzbekistan suspected of links with al-Qaeda, officials said. At least two children also died in the crossfire.
The Taleban have reportedly sent senior commanders to the area to seek a truce.
The area is a stronghold of the Taleban, who have close ties to local tribes.
It is also home to hundreds of foreign militants who fled the US-led invasion of Afghanistan in 2001.
Correspondents say all the militants in the mountainous region want Western troops out of Afghanistan, but divisions and power struggles between them have reportedly grown in recent months.
Uzbek militants had largely kept themselves to themselves and were not linked to al-Qaeda's anti-Western agenda, but in recent months they are reported to have become more involved in local disputes, says the BBC's Aamer Ahmed Khan in Islamabad.
Reports suggested that Taleban and local tribesmen had demanded the Uzbeks leave, or disarm, and that fighting broke out when they refused.
The Pakistani government, which has signed agreements with the tribesmen, has been urging them to kick out al-Qaeda-linked fighters.
Reports say two senior Taleban commanders, Baitullah Mahsud and Khalifa Sirajuddin Haqqani, have gone to the area to try to broker a ceasefire.
Tensions between the mostly Uzbek fighters and local militants spilled over after an Arab militant was killed on Sunday.
The fighting followed similar clashes in which 19 people were killed earlier this month.
Reports from North Waziristan's main town of Miranshah say the dead in the latest fighting included at least 35 foreigners, mostly Uzbeks, and as many as a dozen local militants. Dozens more were wounded.
Pakistan military spokesman Maj-Gen Waheed Arshad confirmed the toll to be over 40.
Teachers said that children were among those killed when a mortar bomb hit their bus.
Local officials said the foreign al-Qaeda militants managed to take away their bodies during a brief truce brokered by local tribesmen.
Fighting was continuing around the villages of Azam Warsak and Kalusha west of the town of Wana on Tuesday night.
The BBC's M Ilyas Khan in Islamabad says the clashes have turned a large area west of Wana into a battlefield, with both sides dug in in several villages.
Thousands of Pakistani troops are in the area
Each side has blamed the other for the outbreak of fighting.
Nato and the Afghan government have criticised Pakistan for recently signing peace deals with militants in South and North Waziristan.
They argue that the agreements have created a safe haven for the Taleban in advance of an expected spring offensive by the militants in Afghanistan.
But the government said the agreements were necessary to reduce the number of Pakistani soldiers who were being killed in the border areas.
More than 700 Pakistani troops have lost their lives in the fight for control over the tribal areas in recent years.