Two members of the U.S.-led military coalition were killed Tuesday by a man wearing an Afghan police uniform in southern Afghanistan, an official said. The killings are the latest in a series of insider attacks that have sown distrust between coalition and Afghan forces.

An Afghan official said the “green-on-blue” attack happened during a joint patrol in Girishk in southern Helmand province. The killer managed to escape, said the official, who spoke by phone on the condition of anonymity.

He said the attacker was a member of the Taliban-led insurgents who have been behind similar attacks in recent years. The killings bring to at least 53 the number of deaths among NATO and U.S. forces in similar attacks this year, the bloodiest period since the phenomenon began in 2007.

In a statement, NATO said the victims were members of the coalition but did not identify them. Most of the foreign troops in Helmand are British and American.

A statement attributed to the Taliban’s reclusive leader Mullah Mohammad Omar last week praised the insider attacks.

The rise in such attacks has strained relations between Kabul and some Western nations. Among measures to combat the surge in insider violence, the coalition and the United States recently restricted joint operations between coalition and Afghan forces and ordered coalition troops to carry loaded weapons at all times.

Dozens of Afghan troops have also been killed in such incidents. The Afghan Defense Ministry said it has arrested hundreds of Afghan troops as part of its effort to curb the violence.

The insider attacks have raised doubts among observers and analysts that Afghan security forces are ready to take over from international troops as the planned transition and elections approach in 2014.

The election is seen as a key factor in Afghanistan’s stability. Amid anxiety in some international and domestic circles about the polling, the country’s Independent Election Commission said Tuesday that the election will be held April 5, 2014, according to an Agence France-Presse report.