The White House condemned a suicide bombing outside the US embassy in Ankara on Friday as a "terrorist attack," but said it did not yet know who was behind it.
"We strongly condemn what was a suicide attack against our embassy in Ankara, which took place at the embassy's outer security perimeter," White House spokesman Jay Carney said.
A Turkish security guard was killed and several other people were wounded in the explosion, which also damaged nearby buildings.
"A suicide bombing on the perimeter of an embassy is by definition an act of terror. It is a terrorist attack," said Carney.
"However, we do not know at this point who is responsible or the motivations behind the attack. The attack itself is clearly an act of terror."
He said the United States was working closely with Turkish authorities "to investigate the incident and bring the perpetrators to justice."
The bombing at a security roadblock near the entrance to the highly-fortified embassy in an upmarket area of the capital was the latest in a series of attacks on American missions in the Muslim world.
Vice President Joe Biden, on a visit to Germany, said the bomber was believed to be a member of an illegal "left wing terrorist organization," without elaborating.