KABUL (AFP) - Investigations were under way on Sunday into the deaths of seven Afghan soldiers and police killed in what Afghan officials have called a mis-targeted NATO air strike.

The probes continued as the military separately announced the deaths on Saturday of two more soldiers, one British and one American, in unrelated incidents.

On Friday four Afghan soldiers and three police officers were killed as NATO and Afghan forces clashed with Taliban insurgents during a search for two missing American paratroopers, officials have said.

"We and NATO are investigating the incident," defence ministry spokesman General Mohammad Zahir Azimi told AFP.

"We know that they were killed in the air strike. We're currently investigating the incident to find out why they were hit, so that such incidents can be prevented in the future," he added.

NATO's International Security Assistance Force (ISAF) said on Sunday only that the probe was continuing and that there was an "ongoing search operation" for the two missing US soldiers.

If proven to be so-called "friendly fire," Friday's incident will have been one of the deadliest of its kind in the eight-year war. Nine Afghan police lost their lives in July last year when NATO fighter planes mistakenly bombed them in the southwestern province of Farah.

ISAF said its personnel and Afghan forces had been "engaged by enemy forces," meaning the Taliban, while searching for the two soldiers who went missing on Wednesday trying to recover airdropped supplies from a river.

It said 25 troops were "killed or wounded during a joint operation that involved multiple engagements over several hours," adding that five American soldiers were among those hurt.

The alliance force said it was "currently investigating whether some of the casualties were caused by ISAF close air support."

Use of air power in Afghanistan has been controversial, with civilian casualties sparking public anger and prompting President Hamid Karzai to demand a halt to air strikes earlier this year.

On Friday, German Defence Minister Karl-Theodor zu Guttenberg said mistakes had been made in a German-ordered air raid in September, carried out by US aircraft, which killed as many as 142 people.

More than 100,000 troops under NATO and US command are in Afghanistan fighting a Taliban insurgency now at its deadliest since US-led troops toppled the Islamist regime in 2001.

A British soldier was killed on Saturday afternoon by a blast in southern Helmand province, Britain's Ministry of Defence said on Sunday, a day known as Remembrance Sunday, when Britain traditionally honours its war dead.

Elsewhere, a US service member died on Saturday in an insurgent attack in western Afghanistan, unrelated to the search for the missing soldiers, ISAF said.

The latest casualties bring to 465 the number of foreign troops killed in the country this year. More than half of the dead are American.

A new poll released Sunday in Britain found further erosion in public support for the war, sparking renewed efforts by officials to explain why Britain must stay the course.

The poll data came days after five other British soldiers died when a "rogue" Afghan policeman attacked a checkpoint in Helmand.

US Secretary of State Hillary Clinton was to leave Sunday for Europe and Asia on a mission partly devoted to consulting with US partners on stabilising Afghanistan, where President Barack Obama's administration is struggling to obtain more support from European partners.

Obama is considering a request from military commanders to boost troop numbers by up to 40,000, as violence continues to rage.

A civilian convoy bringing fuel for international troops was attacked in Baghlan province north of Kabul on Sunday, leaving one driver dead, one wounded and a vehicle in flames, said district chief Amir Gul.

The Afghan defence ministry said Sunday that Afghan troops and NATO war jets also pounded insurgents during a "very fierce battle" in southern Zabul province, killing 17 rebels.

ISAF said "several" militants were also killed after they attempted to attack a convoy of Afghan and international troops in Kandahar province Saturday usin an improvised explosive device.