KABUL: Two American soldiers were killed by a man in an Afghan police uniform on Thursday, the US military said, the latest in a series of insider attacks that have seriously undermined trust between the allies.

The Afghan conflict has seen an alarming surge in insider attacks this year, with more than 50 soldiers from Nato's US-led International Assistance Force (ISAF) killed by their colleagues in the Afghan army and police.

The unprecedented number of so-called “green-on-blue” killings comes at a critical moment in the 11-year war, as Nato forces try to hand security responsibility to the Afghans before the withdrawal of foreign combat forces by the end of 2014. Thursday's attack happened in the central province of Uruzgan.

“Two US Forces-Afghanistan service members died after an individual wearing an Afghan National Police uniform turned his weapon against them in Khas Uruzgan, Uruzgan,” the US military in Afghanistan said.

It is believed to be the first such attack since September 30, when a firefight between Nato troops and their Afghan allies killed five people, including one US soldier and a civilian contractor.

Most insider attacks are blamed on cultural differences and personal grievances between soldiers working closely together against Taliban insurgents, but Nato attributes some 25 per cent of the incidents to Taliban infiltrators.

The attacks have created deep distrust between the allies, and have rocked Nato'S plan to train Afghan forces to take over the fight against the Taliban when foreign troops leave.

NATO top brass have taken the issue very seriously, with ISAF commander General John Allen saying that just as homemade bombs were the signature weapon of the Iraq war, “the signature attack that we're beginning to see is going to be the insider attack”.

US Defense Secretary Leon Panetta told a meeting of Nato defence ministers in Brussels earlier this month that countering the attacks was vital to success in Afghanistan.

Efforts to tackle the phenomenon include orders that Nato soldiers working with Afghan forces should be armed and ready to fire at all times, even within their tightly protected bases, and the issuing of cultural guidelines.

The guidelines, drawn up by the Afghan defence ministry, urge their soldiers not to take offence if Nato colleagues exit the shower naked, swear or ask to see pictures of their wives.

The 28-page brochure tells Afghan soldiers these things are normal behaviour and no reason to open fire.