Five Afghan soldiers have accidentally been killed in a Nato airstrike, officials in Afghanistan have said.

A spokesman for the Afghan defence ministry condemned the incident, saying it was not the first time Afghan soldiers had died in "friendly fire".

Gen Mohammad Zahir Azimi said the soldiers had been launching an attack against insurgents in Ghazni province in eastern Afghanistan.

Nato confirmed the airstrike had gone wrong and said it regretted the deaths.

Spokesman Brig-Gen Josef Blotz said a joint investigation had been launched.

A statement released by the the Nato-led International Security Assistance Force (Isaf) said that the inquiry would determine the facts and circumstances surrounding the deaths, which occurred when "an Isaf aircrew engaged the individuals with precision-guided munitions".

"This loss of life is tragic, and we offer condolences to all those who lost loved ones," said Isaf spokesperson Jane Campbell.

"We work extremely hard to co-ordinate and synchronize our operations, and we deeply regret the loss of lives from our Afghan partners."

Isaf has also extended the personal condolences of US Gen David Petraeus, the new commander of Nato forces in Afghanistan, to families of the victims.

The incident comes as international forces try to improve co-ordination with their Afghan counterparts in order to eventually transfer more security operations to them.

Ambush

Gen Azimi said the soldiers had been launching a morning ambush on militants in the Sarda dam and Rahim Khail village area when a Nato aircraft opened fire on them without warning.

He said two other soldiers were wounded in the attack.

"We condemn this action. Unfortunately this is not the first time such an incident has happened, but we wish that at least this would be the last one," Gen Azimi said.

Civilian casualties and "friendly fire" incidents involving Afghan troops have been a frequent source of friction between Western powers and Afghan President Hamid Karzai.

Image caption Brig Gen Josef Blotz expressed Nato's regret for the incident

Former US and Nato commander Gen Stanley McChrystal made avoiding civilian casualties a priority in a policy known as "courageous restraint".

His successor, Gen Petraeus, will not abandon the policy but, according to senior sources, he believes some units are interpreting its rules too cautiously.

About 140,000 international troops are fighting alongside Afghan forces to quell a Taliban-led insurgency.

The fiercest fighting is taking place in southern Afghanistan, which is the focus of a new US-led push to regain control of militant-held areas.

In other developments, Nato said three US soldiers were killed by a roadside bomb in southern Afghanistan on Tuesday. It did not give any further details.

Britain announced on Wednesday that it would withdraw its troops from the Sangin valley in the southern Helmand province, handing over responsibility to US forces.