Five US soldiers in eastern Afghanistan were wounded when an individual in Afghan army uniform opened fire on them on Tuesday, officials say.

The five are receiving medical treatment. The severity of their injuries is not yet known.

The attack took place at a Nato base in Wardak province, east of Kabul.

It is the latest in a series of so-called "green-on-blue" attacks, where Afghans in the police or army attack international forces.

The AFP news agency says there have been 19 such attacks so far in 2012.

On Sunday, three British soldiers were shot and killed by an Afghan policeman as they left a checkpoint in Helmand province, in the south.