KABUL (Reuters) - An Afghan wearing a uniform killed three U.S. soldiers in the southern province of Helmand, a spokeswoman for NATO-led forces in Afghanistan said on Friday.

"All we know is that they were killed by an Afghan in a uniform of some sort," the spokeswoman told Reuters, adding that it was too early to say if the shootings were by a rogue security force member or a Taliban infiltrator.

Afghan officials said the three men were all special forces members and were killed while attending a meeting in Sangin District late on Thursday.

So-called green on blue shootings, in which Afghan police or soldiers turn their guns on their Western mentors, have seriously eroded trust between the allies as NATO combat soldiers prepare to hand over to Afghan forces by 2014, after which most foreign forces will leave the country.

According to NATO, there have been 24 such attacks on foreign troops since January in which 28 people have been killed. Last year, there were 21 attacks in which 35 people were killed.

In a grim 24 hours for the NATO-led force, three U.S. soldiers and an American aid worker were killed earlier on Thursday in the eastern province of Kunar in an attack by a suicide bomber.