The White House on Thursday denounced an attack at a hospital in Afghan's capital of Kabul, in which three American doctors were shot dead.

"Any such attack on civilians at a hospital is despicable and cowardly," National Security Council spokeswoman Caitlin Hayden said in a statement.

She vowed continued U.S. support for the Afghans working to build a "peaceful, prosperous future for themselves."

The Americans were killed by an Afghan security guard, whose motive was not known yet. A female health worker was injured in the shooting.

In a similar attack on Kabul's Serena Hotel on March 20, nine civilians including five foreigners were killed.

Attacks on foreigners have been on the rise in Afghanistan this year, prompting the United Nations and other international organizations to boost their security.

The American and NATO troops are preparing for an exit from Afghanistan by the end of this year after invading the country in October 2001.