Four Afghan policemen were shot dead by their colleagues at a police checkpost in southern Afghanistan, officials said Friday.

The shooting, which is the latest in a series of Afghan security personnel targeting their local and foreign colleagues, occurred in Greshk district of volatile Helmand province late Thursday evening. "Four policemen were killed by gunshots by their five colleagues in a police checkpost last night around 10:00 PM," Tooryalai, police chief of Greshk district in Helmand province, told AFP.

The five attackers managed to flee after the killing, Tooryalai said. The five gunmen had been sent to the checkpost as reinforcements to prevent a possible Taliban attack before opening fire on their colleagues, he said. Both the attackers and the dead were new recruits to the police force, Tooryalai added.

Abdul Nabi Elham, police chief of Helmand, confirmed the death toll and said an investigation had been launched to determine whether there was any "verbal dispute" between the two sides or whether Taliban infiltrators were behind the shootings.

There was no claim of responsibility for the killing, but similar attacks have been carried out in the past by Taliban insurgents who have infiltrated the Afghan security forces.

Afghan police are the key force in supporting US-led NATO forces in a war against Taliban militants. Afghanistan has witnessed a surge in insider attacks, often by Taliban spies, which have killed more than 50 foreigners and dozens of local security personnel this year.

The unprecedented number of insider killings comes at a critical moment in the 11-year war, as US-led NATO forces plan to hand security responsibility to the Afghans ahead of a withdrawal of troops by the end of 2014.