(Adds that Americans were Marine special operations forces)
By Abdul Malik
LASHKAR GHAR, Afghanistan, Aug 10 (Reuters) - An Afghan police commander and several of his men killed three U.S. Marines in the southern province of Helmand, turning guns on them after inviting them to a dinner to discuss security, Afghan officials said on Friday.
The men were all Marine Corps special operations forces and were killed on Thursday night while attending a meeting in the Sarwan Qala area, in what appeared to be a planned attack by rogue Afghan forces.
"During dinner, the police commander and his colleagues shot them and then fled. The commander was Afghan National Police in charge of local police in Sangin," a senior Afghan official told Reuters on condition of anonymity. Sangin is a district.
"It looks like he had drawn up a plan to kill them previously," the official said.
A spokeswoman for NATO-led forces in the country confirmed the incident but said it was too early to say whether it was a rogue shooting or due to insurgent infiltration.
"All we know is that they were killed by an Afghan in a uniform of some sort," the spokeswoman said.
The Pentagon said later the slain servicemen were Marine Corps special operations forces, but it was unable to confirm other details of the shooting.
So-called green on blue shootings, in which Afghan police or soldiers turn their guns on their Western colleagues, 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.
NATO says there have been 24 such attacks on foreign troops since January in which 28 people have been killed, not including Thursday's attack. Last year there were 21 attacks in which 35 people were killed.
Another foreign soldier was killed in the south on Friday during an insurgent attack, NATO said, while seven civilians were killed and three were wounded by an insurgent roadside bomb, also in Helmand.
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. (Additional reporting by Rob Taylor and Mirwais Harooni; Writing by Rob Taylor; Editing by Robert Birsel and Xavier Briand)