The search was halted as darkness fell and was likely to resume in the morning, the forces said.

The chopper last made contact with air traffic controllers as it was about to leave Pakistan air space on Friday to fly over Afghanistan on the way to its home base in Turkmenistan, the International Committee of the Red Cross said.

"Since then, we have had no contact," said James Reynolds, a spokesman in Pakistan for the Red Cross which chartered the chopper after the October 8 earthquake that killed thousands of Pakistanis.

The NATO peacekeeping force in Afghanistan dispatched "several" military helicopters and fixed-wing aircraft to search the mountainous border with Pakistan and with Turkmenistan, spokeswoman Annie Gibson-Sexton said.

The US-led coalition, which has been here for four years to hunt down militants from the ousted Taliban regime, also had aircraft in the search, spokesman Mike Cody said without giving details.

"The coalition will resume the search in the morning," Cody said.

Pakistani forces also conducted land and air searches on Saturday and Sunday in the areas bordering Afghanistan.

Pakistani military spokesman Brigadier Shahjahan Ali Khan said the aircraft might have come down on the rugged Afghan border.

"The helicopter might have crashed astride the border. It is our guess, we have no clue so far," he told AFP, adding that the helicopter had been due to refuel in Afghanistan.

The Mi-8 transport helicopter left the Pakistani city of Peshawar on Friday after completing its relief work, the Red Cross's Reynolds said.

It was carrying seven crew members from Turkmenistan but no Red Cross staff.

"We have no information as to the fate of the crew. We are in contact with the authorities in Pakistan, Afghanistan and Turkmenistan," Reynolds told AFP.

The chopper, which arrived in Pakistan on October 23, was operated by Turkmenistan Airlines.

"We are constantly trying to make contact with the helicopter. We are trying to reach Kabul but the connection is very bad," an airline official told AFP.

The helicopter was one several international aircraft providing essential rescue and relief help after the earthquake that killed more than 73,000 people in mountainous northwestern Pakistan and Pakistani Kashmir.

About 3.5 million others lost their homes and the United Nations has warned that hundreds of thousands of people living in tents are at risk of falling ill during the bitter Himalayan winter.

Six Pakistani soldiers were killed when a Pakistan army Mi-17 helicopter crashed during relief operations in Kashmir on October 16.

More than a dozen helicopters operating under NATO and coalition deployments have crashed or been shot down by insurgents since the ousting of the Taliban regime in 2001.

In April last year an American Chinook helicopter crashed in bad weather in the southern province of Ghazni, killing all three civilians and 15 soldiers aboard.

It was the worst helicopter crash during US operations in Afghanistan.

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