A suicide bomber has carried out an attack at an entrance to the US embassy in the Turkish capital Ankara, killing himself and a guard.
The bomber's device went off as he was going through an X-ray machine at a security gatehouse, Sky sources said.
The White House condemned the suicide bombing as a "terrorist attack," but said it did not yet know who was behind it.
White House spokesman Jay Carney said "we strongly condemn" the attack, adding: "A suicide bombing on the perimeter of an embassy is by definition an act of terror. It is a terrorist attack.
"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."
Damage can be seen at a side entrance of the embassy Turkey's Prime Minister said an outlawed Turkish Marxist group was responsible for the attack.
Tayyip Erdogan said on Turkey's HaberTurk television that it was the work of the Revolutionary People's Liberation Party-Front, or DHKP-C. The group has been designated a terrorist organisation by the United States and the European Union.
Mr Erdogan said: "It is my principle to speak only after receiving the final result, but I can say clearly that it is DHKP-C."
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. There was no immediate claim of responsibility for Friday's attack.
A Turkish guard was killed in the blast and another Turkish citizen, believed to be a journalist, was wounded.
Medics treat a woman who was injured in the blast 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.
"We can confirm a terrorist blast at a checkpoint on the perimeter of our embassy compound in Ankara, Turkey, at 1.13pm local time, or 6:13 am ET," spokeswoman Victoria Nuland 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," she added.
Foreign Secretary William Hague condemned the bombing, and UK businesses in Turkey have been urged to be vigilant.
The British consulate in Istanbul said in a statement: "There has been a suspected terrorist attack on the US embassy in Ankara.
"Due to the nature of this incident we advise you to be extra vigilant and to take appropriate security measures to safeguard your staff and assets."
Ilnur Cevik, a journalist who was near the scene, told Sky News: "There was a huge bang which really shook everywhere."
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.
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