LONDON (AFP) - The Taliban have claimed the killing of five British soldiers by a policeman in Afghanistan, Prime Minister Gordon Brown said on Wednesday, adding that five other troops were also seriously injured.

"What we know is that the Taliban have claimed responsibility for this," Brown told lawmakers.

"It may be that the Taliban have used an Afghan police member or that they have infiltrated the Afghan police force," he added, saying it was a "terrible and tragic incident".

Opposition Conservative leader David Cameron said the fact that the gunman had been part of a training programme conducted by his victims raised serious questions about the safety of troops mentoring Afghan security forces.

But Brown responded that although security would be tightened, the training programmes were crucial because they showed that international forces were helping Afghanistan to manage its own affairs.

"Security will be stepped up where that is necessary, but we cannot desist from the practice which is absolutely essential for the future of Afghanistan and the security of our country and that is training and mentoring the Afghan forces," he said.

"That means that our troops will be working with the Afghan police and the Afghan army," he added.

"It is an essential element of the whole coalition strategy that we train up the Afghan forces so that they themselves are able to take over the security of this country, and that we will continue to do."

The soldiers died of gunshot wounds in the attack at a checkpoint in the Nad-e'Ali district of Helmand Province on Tuesday. They had been mentoring Afghan police and living inside the checkpoint.

An Afghan military spokesman said the gunman had been a "rogue" policeman, possibly acting in conjunction with another.

A statement from the NATO-run International Security Assistance Force (ISAF) gave the number of wounded soldiers as six.