US forces in Afghanistan carried out a failed attempt to rescue two hostages last month, the Pentagon has revealed.
A spokesman said a number of "hostile forces" were killed, but the hostages were not at the location.
He said no US service personnel or civilians were harmed.
No details were given about the hostages but US media reported that they were two professors, one from the US and one from Australia, abducted in Kabul in early August.
The two men worked for the American University of Afghanistan.
Pentagon spokesman Peter Cook said the rescue attempt had been authorised by US President Barack Obama on the recommendation of Defence Secretary Ash Carter.
"Unfortunately, the hostages were not at the location we suspected," Mr Cook said.
"During the mission, US forces engaged and killed a number of hostile forces."
The two professors had been on their way to their guesthouse when gunmen wearing uniforms of the Afghan security forces attacked their vehicle on 7 August, reports said.
It is not known which group was responsible for the abduction or if a ransom has been sought.
Several foreigners have been attacked or kidnapped in Afghanistan this year.