(Reuters) - Three foreign soldiers were shot dead Wednesday by a gunman wearing an Afghan army uniform in southern Afghanistan, the NATO-led coalition said, the latest in an alarming series of insider shootings which have damaged trust between the allies.

NATO sources said the soldiers were killed in Uruzgan province, where around 1,500 Australian troops are based.

"The incident is currently under investigation," the NATO-led International Security Assistance Force said in a Thursday morning statement giving no other details.

The deaths bring to 15 the number of foreign soldiers killed this month in insider attacks, which has prompted NATO to increase security to try prevent them, including requiring soldiers to carry loaded weapons at all times on base.

U.S. Joint Chiefs of Staff chairman Martin Dempsey visited Kabul this month to talk about rogue shootings and urged Afghan officials to take tougher preventative action, although Western commanders have ruled out cutting back training and support between NATO and its Afghan allies.

Including Wednesday's shooting, this year has seen 34 insider attacks, called green on blue incidents, leading to 45 coalition deaths. This accounts for 14 percent of all NATO deaths this year in Afghanistan.

That is a sharp increase from 2011, when, during the whole year, 35 coalition troops were killed in such attacks, 24 of whom were American.

Afghanistan's government said last week it would re-examine the files of 350,000 soldiers and police to help curb rogue shootings of NATO personnel, but accused "foreign spies" of instigating the attacks, in a swipe at Pakistan and Iran.