KHAR, Pakistan - Pro-government tribesmen killed 23 militants in clashes in Pakistan's northwest in the latest violence between tribal militias and Taliban insurgents, a government official said Tuesday.

Elsewhere in the volatile northwest, suspected militants attacked an oil tanker carrying fuel to NATO forces in Afghanistan, and Taliban fighters exchanged fire with U.S. and Afghan troops across the border in Afghanistan, officials said.

The clashes between militants and tribesmen took place in the village of Ambar in the Mohmand region, part of the lawless tribal belt along Pakistan's border with Afghanistan where top Taliban and al Qaeda leaders are believed to be hiding.

Syed Ahmad Jan, a senior regional administrator, said local tribal militia asked the militants to leave the area late Monday. The militants refused and opened fire, sparking a gunbattle that was still raging Tuesday morning, Jan said. Four tribal militiamen were wounded in the fight.

Pakistan's government has encouraged tribesmen in the semiautonomous frontier region to form local militias -- known as lashkars -- to repel Taliban militants blamed for attacks in Pakistan and Afghanistan.

Such groups have been set up in several regions but face stiff Taliban resistance.

The ambush on the oil tanker took place in the nearby Khyber region, said Fazal Mahmood, a local government official.

The attackers opened fire on the truck, then fled when the security forces escorting the convoy returned fire, Mahmood said. The truck driver and a passer-by were killed. The tanker caught fire in the attack.

Taliban militants have frequently targeted NATO supply convoys in the region, home to the Khyber Pass, a major land transit route for U.S. and NATO supplies into Afghanistan.

Meanwhile, intelligence officials said Pakistani militants exchanged fire with U.S. and Afghan troops in Afghanistan.

The Pakistani Taliban, operating from the South Waziristan tribal region, fired three rockets across the border and the American and Afghan forces retaliated with eight mortar shells, two officials said.

A civilian was wounded by the fire from the Afghan side, the officials added, speaking on condition of anonymity because they were not authorized to talk to the media.

An army spokesman could not immediately be reached for comment. It was not possible to independently verify the information because access to the remote, dangerous region is strictly controlled.

Pakistan's military is readying its latest major offensive against Taliban fighters in South Waziristan to the south. The U.S. strongly supports Islamabad's efforts and believes they could help the U.S.-led war in Afghanistan.

But Pakistan also has to grapple with a massive refugee crisis in the northwest sparked by another major offensive in the Swat Valley.

After weeks in sweltering camps, refugees from the valley began heading home Monday -- the first day of the government's official repatriation program for those uprooted by fighting there. More were making the trip Tuesday.

Some refugees, however, have refused to go back, fearing for their safety and demanding aid promised by the government. Thousands more who tried to return without official permission were blocked by the military.

The repatriation program's sputtering start illustrates the Pakistani government's struggles to respond to one of the most challenging humanitarian crises in the country's history.

The government has sought to downplay the concerns.

Amir Haider Khan Hoti, the chief minister of the North West Frontier Province, assured refugees Monday the government was strengthening the police force to help keep out the Taliban. The army has already said it expects to stay in Swat for another year.