A leftist group has said it was responsible for a suicide attack at an entrance to the US embassy in the Turkish capital Ankara, which killed the bomber and a guard.
The attacker's device went off as he was going through an X-ray machine at a security gatehouse, Sky sources said.
Turkish prime minister Tayyip Erdogan had earlier pointed the finger at the Revolutionary People's Liberation Party-Front.
On Saturday the group - also known as DHKP-C - claimed responsibility in a statement on its website.
The White House has condemned the suicide bombing as a "terrorist attack".
White House spokesman Jay Carney said: "A suicide bombing on the perimeter of an embassy is by definition an act of terror. It is a terrorist attack."
The perpetrator had reportedly spent time in prison on terrorism-related charges. He was believed to be aged about 30 and used plastic explosives in the embassy atrocity.
The DHKP-C is a Marxist-Leninist group that has claimed responsibility for suicide bombings against police stations in recent years.
The group has been designated a terrorist organisation by the United States and the European Union.
A Turkish guard was killed in the blast and another Turkish citizen, believed to be a journalist, was wounded.
TV footage showed a door blown out at a side entrance and there was scattered masonry from a wall, although there did not appear to be any more significant structural damage.
Police cordoned off the area following the explosion, which sent smoke and debris flying into the street.
Americans were warned to avoid visiting the embassy or US consulates in Istanbul and Adana until further notice, and were told to register on the State Department's website.
"The Department of State advises US citizens travelling or residing in Turkey to be alert to the potential for violence, to avoid those areas where disturbances have occurred, and to avoid demonstrations and large gatherings," said a statement by the US Consulate in Istanbul.
The State Department said the explosion occurred on the perimeter of the embassy.
A spokeswoman said: "We are working closely with the Turkish national police to make a full assessment of the damage and the casualties, and to begin an investigation."
Foreign Secretary William Hague also condemned the bombing, and UK businesses in Turkey have been urged to be vigilant.
Far-left groups in Turkey oppose what they see as US influence over Turkish foreign policy.
Turkey is a key US ally in the Middle East with common interests ranging from energy security to counter-terrorism.