A suicide bomber suspected to be a militant from an outlawed leftwing group blew himself up at the US embassy in Ankara on Friday, killing a Turkish security guard and wounding three other people, officials said.
The bombing at 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, highlighting the vulnerability of the country's 70,000 diplomats.
The White House strongly condemned the bombing, with spokesman Jay Carney saying: "The attack itself is clearly an act of terror."
"However, we do not know at this point who is responsible or the motivations behind the attack."
Turkey's Interior Minister Muammer Guler told reporters that the bomber was believed to be "a militant of a banned leftist organisation," without elaborating.
"We lost one of the three guards at the entrance, while the two others survived with injuries," he said, adding that a female journalist was also seriously wounded.
Local media identified the bomber as a member of the Revolutionary People's Liberation Front (DHKP-C), who had been jailed after a 1997 attack at a military compound in Istanbul.
Two weeks ago, Turkey carried out a major nationwide crackdown on the DHKP-C, a Marxist group blamed for several attacks since the late 1970s including suicide bombings but Guler did not confirm it was responsible.
DHKP-C is vehemently anti-US, anti-NATO, and anti-Turkish establishment.
However, there was no immediate claim of responsibility for what was the latest of many bloody attacks in Turkey which in the past have been blamed on Kurdish militants, leftist extremists or Al-Qaeda linked groups.
Friday's bombing came on the last day of Hillary Clinton's tenure as US secretary of state and a week after NATO declared that a battery of US-made Patriot missiles went operational on Turkey's border with war-torn Syria.
"The attacks target the well-being and peace in our country," Prime Minister Recep Tayyip Erdogan said in televised remarks. "We will stand tall and we will stand together... we will get over these."
The force of the blast damaged nearby buildings in the Cankaya neighbourhood of the capital where many other state institutions and embassies are also located.
Police cordoned off the area, while a police helicopter hovered in the air above and armed US Marines patrolled the embassy roof.
Television footage showed the wounded journalist with a blood-stained face being carried into an ambulance on a stretcher.
US ambassador Francis Ricciardone vowed to work with Turkey to fight terror, confirming the death of the Turkish security guard and saying: "The compound is secure."
"We will continue to fight terrorism together. From today's event it is clear we both suffer from this terrible terror problem," he told reporters. "We are determined... only more to collaborate together until we defeat this problem."
The embassy warned US citizens to avoid its diplomatic missions in Turkey until further notice and to avoid potential troublespots and demonstrations.
In September, the US ambassador to Libya and three other people were killed when dozens of heavily-armed Al-Qaeda-linked militants overran the US consulate in the eastern city of Benghazi and a nearby CIA-run annex.
French Foreign Minister Laurent Fabius and his British counterpart William Hague condemned the Turkish bombing while German Chancellor Angela Merkel offered her sympathies to the victims.
Predominantly Muslim Turkey, a close US ally and NATO member, has become a fierce critic of the regime of President Bashar al-Assad since the uprising erupted in March 2011 and called for the deployment of the Patriot missiles after several cross-border attacks.
Western missions in Turkey have been targeted in the past.
In July 2008, three gunmen and three Turkish policemen were killed in an attack outside the US consulate in Istanbul.
In November 2003, four suicide car-bomb attacks on two Istanbul synagogues, the British consulate and British bank HSBC killed 63 people, including Britain's consul general. They were claimed by an Al-Qaeda cell.
Friday's attack also came as the Turkish government is negotiating with leaders of the outlawed Kurdistan Workers' Party (PKK) to resolve the three-decade Kurdish conflict.
The insurgency by Kurdish rebels seeking autonomy in the southeast has claimed 45,000 lives, most of them Kurdish.
The PKK, which is regarded as a terrorist group by Ankara and its Western allies, had stepped up its attacks last year, usually targeting Turkish security forces.