HERAT, Afghanistan (AFP) - - Nine policemen were killed in Afghanistan Sunday in international military air strikes called in when police and troops clashed after mistaking each other for Taliban, authorities said.

The "friendly fire" incident occurred before dawn when Afghan and international soldiers moved into a district in the southwest without informing police, who thought they were militants, the deputy governor of Farah province said.

"An engagement took place, each side thinking the other was the Taliban," said Mohammad Younus Rasouli.

The troops called for air support and military attack aircraft arrived and bombed a police post, he said. Nine police were killed and five wounded.

Rasouli said NATO's International Security Assistance Force had carried out the strikes, but ISAF said it was an operation by the separate US-led coalition. The coalition confirmed an incident and said it was investigating.

Rasouli said the police chief of Farah's Anar Dara district, on the border with Iran, was among the wounded and was in a serious condition.

The incident comes as US presidential hopeful Barack Obama is visiting Afghanistan to find out how international efforts against extremist militants trying to overthrow the Afghan government are progressing.

There have been several deadly incidents of "friendly fire" in Afghanistan, where many local and international security forces are involved in a growing fight against Taliban insurgents.

Earlier this month a British helicopter mistakenly opened fire on a group of British soldiers in Helmand province, injuring nine of them, three seriously, the defence ministry said.

In January nine Afghan policemen were killed in the central province of Ghazni by US-led soldiers hunting militants, Afghan officials said.

ISAF meanwhile said Sunday that its soldiers had killed four Afghan civilians by accident when mortar rounds landed off target in the eastern province of Paktika near the border with Pakistan.

"An ISAF unit on a fire mission accidentally killed four civilians, with an unconfirmed further three deaths," it said in a statement. "Four civilians were also wounded and are now under treatment by ISAF forces."

It is the latest incident in which the international soldiers helping the Afghan government have killed civilians by mistake.

The US-led coalition admitted last week that it had killed eight civilians in an air strike targeting militants in Farah. Afghan officials said nine women and a boy were killed.

The coalition and ISAF are also investigating official Afghan reports that 64 civilians were killed in two strikes in northeastern Afghanistan early this month.

One hit a wedding party, killing 47 people including the bride, an investigation appointed by President Hamid Karzai found.

Civilians are regularly caught in the crossfire of the insurgency launched after the hardline Islamic Taliban regime was removed from power in late 2001 in a US-led invasion for harbouring Al-Qaeda.

Most are killed in rebel attacks, but dozens have also been killed in military action this year.

On Sunday, three children were killed in the southern province of Hemland when a bomb blew up a minivan, provincial police chief Mohammad Hussien Andiwal said. Two children and two adults were wounded.

Taliban fighters attacked a police post in the same area around midnight and two of the attackers were killed in the ensuing fight, Andiwal said.

The defence ministry said in a statement meanwhile that its soldiers had killed nine "terrorists" in an operation to secure the road between Kabul and Kandahar, a dangerous route along which Taliban are active.

And 18 were killed in a week of action in Kandahar province's volatile Panjwayi district, it said.