A renegade Afghan soldier killed three British army Gurkha soldiers during a joint patrol on Tuesday in southern Helmand province, officials said.

The man responsible for killing the troops was still on the run, but the military has been making "strenuous efforts" to find him, two statements said.

The troops killed were Gurkhas, who are drawn from Nepal. The attack happened near Lashkar Gah, the provincial capital of Helmand, where some 9,000 British troops are based as part of the NATO-led force.

NATO said in a statement that three of its soldiers were killed in an attack in southern Afghanistan, but gave no further details.

"The soldiers were killed in a suspected premeditated attack by a member of the Afghan National Army using a combination of weapons," the Ministry of Defence said in a statement.

A spokesman for Task Force Helmand, Lieutenant Colonel James Carr-Smith, confirmed the attack.

"It is with deep sorrow I must inform you that three soldiers from the 1st Battalion The Royal Gurkha Rifles Battle Group were killed this morning in southern Nahr-e Saraj, Helmand Province," he said.

"We believe these were the actions of a lone individual who has betrayed his ISAF and Afghan comrades."

"We heard about this this morning with regret and the President was upset to hear this," Waheed Omer, spokesman for Afghan president Hamid Karzai, said.

British prime minister David Cameron said he was "appalled" by the attack.

The attack is not the first time foreign troops have been killed by renegade Afghan security forces, raising concern among some in the West about the degree of Taliban infiltration in state security forces trained and funded by the NATO as part of its fight against the resurgent militants.

An Afghan policeman killed five British soldiers at a training base in southern Helmand province last November.

A month later, an Afghan soldier shot dead a US service member and wounded two Italian soldiers in a joint NATO and Afghan base in north-western Badghis.

There have been several other attacks by men in army and police uniforms against government and international forces.

- AFP/Reuters