Britain's defense ministry says an Afghan man dressed in a police uniform killed three British soldiers Sunday.

In a statement Monday, the ministry said the incident occurred at a checkpoint in southern Afghanistan's Helmand Province.

Defense officials say the soldiers were part of a police advisory team that was meeting with local elders. As the team left the checkpoint, a man wearing an Afghan police uniform opened fire. The three soldiers died despite receiving first aid at the scene.

British Defense Minister Philip Hammond called the incident “deeply tragic” but promised that those types of attacks would not “derail” or “distract” Britain in Afghanistan.

NATO still is investigating whether the attacker was a police officer or insurgent.

There have been at least 18 so-called “green-on-blue” incidents this year resulting in the deaths of 23 NATO soldiers.

The growing number of these attacks has raised questions as NATO works to transfer full security responsibility of the country to Afghan forces by 2014.

U.S. data released earlier this year showed that at least 60 percent of the attacks are motivated by personal issues.