A Briton and two Americans were killed in a shooting at a training centre in Herat, western Afghanistan, on Sunday, the BBC has been told.
Earlier a Nato spokesman said a man in Afghan national security force uniform had fatally shot the three contractors.
The gunman had been killed and the matter was being investigated, he said.
Local officials said an Afghan National Army soldier also shot and injured two US soldiers in the northern province of Faryab on Monday before being killed.
They said the shooting took place inside headquarters in the district of Ghormach - home to a joint ANA and ISAF base.
Five Nato service members were also killed in roadside bombings over the weekend.
On Sunday, Afghan officials said the gunman in Herat also injured the contractors' Afghan translator. The attacker was said to have worked for the regional police command.
The BBC's Bilal Sarwary, in Kabul, said hundreds of Afghan recruits from several western provinces had joined the national police at the Herat base.
Some of the recruits came from remote areas where Taliban insurgents were in control, he said.