"Three International Security Assistance Force service members died when an individual wearing an Afghan National Security Forces uniform shot them in eastern Afghanistan today," Khaama Press reported the NATO-led International Security Assistance Force said in a statement.
While the ISAF didn't release details about the soldiers killed or the shooter, CNN reported officials said they were members of the U.S. Army Special Operation Forces who were taking part in a training exercise when the assailant turned his gun on them. It wasn't known whether the shooter was a member of the Afghan army or had stolen a uniform.
Officials said other soldiers in the area quickly killed the assailant, the U.S. network said.
Gen. Zahir Azimi, an Afghan Defense Ministry spokesman, said the attack took place in Gardez, the capital of Paktia province, Khaama Press said.
The last reported insider attack was July 9, when an Afghan soldier at a military base in Kandahar shot dead one soldier and wounded three others.
No group had claimed responsibility for the latest attack, though the Taliban has used the tactic in the past.