Afghan officials announced meanwhile that at least a dozen Taliban fighters lost their lives in incidents overnight, and bombing raids in the southern province of Helmand were believed to have killed and wounded several more.

The attack on the police was one of the deadliest on the fledgling force, which is regularly hit by ambushes and roadside bombs in an insurgency led by the extremist Taliban movement that was driven from government in 2001.

A three-vehicle police convoy was on its way from the south to the capital, Kabul, when it was ambushed along the main highway -- one of the busiest in the country, the interior ministry said.

"Sixteen police were martyred and another six police were wounded today at 8:30 am in an ambush by the enemies of peace in Afghanistan," spokesman Zemarai Bashary told AFP.

There were also casualties on the Taliban side, Bashary said, without giving a number.

Nato's International Security Assistance Force (Isaf) said in Kabul meanwhile that a Chinook that came down in Helmand late Wednesday may have been hit by hostile fire. The Taliban movement said its men shot down the chopper.

"We are able to say now that there may have been enemy fire in bringing down the helicopter," the Isaf media office in Kabul said. "It could be anything from small arms fire upwards."

The 37-nation force does not release the nationalities of its casualties, but the Ministry of Defence announced in London that one of the seven dead was a British soldier.

The icasualties.org website, which charts casualties in the operations in Afghanistan and Iraq, said five of the dead were US nationals, one Canadian and a Briton.

Taliban spokesman Yousuf Ahmadi said late Wednesday that "our brothers in Helmand" had brought down the heavy transport helicopter.

"The helicopter burst into flames in the sky and then crashed," he said, citing the local rebel cell that claimed to have to carried out the attack.

The insurgents have made similar claims in the past but are only confirmed to have brought down one military aircraft in Afghanistan -- in June 2005 when a rocket-propelled grenade shot down a Chinook in Kunar province.

All 16 soldiers on board, eight of them US Navy SEALs, were killed.

The defence ministry said meanwhile that Afghan and ISAF soldiers became involved in heavy fighting in Helmand early Thursday.

"Tens of enemy elements have been killed and wounded," it said in a statement. "Various enemy locations were bombed by the air forces and the operation is still ongoing."

Hardcore Taliban are said to be allied with drugs barons and foreign fighters in Helmand, the top producer of Afghanistan's illegal opium.

The province is where the Afghan and foreign military forces said they killed the Taliban's top military commander, Mullah Dadullah, this month in the military's biggest success against the movement.

The rebels have vowed to avenge the commander's death.

Separately the district centre in western Farah province, which is on the Iranian border, came under "heavy attack by enemies of peace and sovereignty" overnight, the interior ministry said Thursday.

Police returned fire, killing 10 rebels and injuring 15, it said.

And in the southern province of Kandahar, two Taliban insurgents fell victim to one of their own bombs when the device exploded as they were planting it, Panjwayi district governor Niaz Mohammad Sarhadi said.