An Afghan policeman opened fire Thursday on staff at a Kabul hospital run by a US charity, killing three Americans.

An Afghan policeman opened fire Thursday on staff at a Kabul hospital run by a US charity, killing three Americans.

The gunman was injured in the incident outside the CURE International hospital and detained by police, officials said, adding that the motive behind the shooting was not immediately known.

"The attacker was a member of the police in the district," Seddiq Sediqqi, spokesman for the interior ministry, told AFP.

"He opened fire as the foreign nationals were entering the hospital, tragically killing three and injuring one more. Another policeman in the area shot the attacker, injuring him."

A medic who was treating the wounded told AFP that a foreign doctor was among the dead.

"With great sadness we confirm that three Americans were killed in the attack on CURE Hospital," the US embassy said on its Twitter account. "No other information will be released at this time."

Cure International is a non-profit organization founded in 1998, based in Pennsylvania and working in 29 countries including Afghanistan.

It describes itself as an "unapologetically Christian organization" on its website, adding that it offers "treatment regardless of gender, religion, ethnicity, or ability to pay".