An intense search on Sunday involving several choppers and planes was called off as darkness fell, 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 eastern border with Pakistan and with Turkmenistan in the north, a spokeswoman said.

The US-led coalition also had aircraft in the search, spokesman Mike Cody said. "The coalition will resume the search in the morning," Cody said Sunday.

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

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

Pakistani forces conducted land and air searches at the weekend to no avail.

The Mi-8 transport helicopter left the Pakistani city of Peshawar on Friday. It was carrying seven crew members from Turkmenistan but no Red Cross staff.

The helicopter was one of several international aircraft that provided 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.

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