A suicide bomber who attacked the U.S. Embassy in Ankara on Friday, killing himself and a security guard, belonged to an illegal leftist group, Turkish Interior Minister Muammer Guler said.
Speaking to reporters, Guler said the attacker could have been a member of the militant Revolutionary People's Liberation Party-Front (DHKP-C) or another leftist group.
The U.S. consulate in Istanbul warned its citizens against visiting its missions in Turkey until further notice.
"The Department of State advises U.S. citizens traveling 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," the consulate statement added.
Ankara Governor Alaaddin Yuksel said the attacker was inside U.S. property when the explosives were detonated. The blast sent masonry spewing out of the wall of the side entrance, but there did not appear to be any more significant structural damage.
The bomber was also killed.
U.S. Ambassador Francis Ricciardone emerged through the main gate of the building, which is surrounded by high walls, shortly after the explosion to address reporters, flanked by a security detail as a Turkish police helicopter hovered overhead.
"We are very sad of course that we lost one of our Turkish guards at the gate," Ricciardone he said, thanking the Turkish authorities for a prompt response.
A Reuters witness saw one wounded person being lifted into an ambulance as police armed with assault rifles cordoned off the area.
"It was a huge explosion. I was sitting in my shop when it happened. I saw what looked like a body part on the ground," said travel agent Kamiyar Barnos whose shop window was shattered around 100 meters away from the blast.
One witness said the blast was audible a mile away.
There was no immediate claim of responsibility. The British Consulate-General to Turkey said the blast a "suspected terrorist attack".
Islamist radicals, far-left groups, far-right groups and Kurdish separatist militants have all carried out attacks in Turkey in the past.
The main domestic security threat comes from the separatist Kurdistan Workers Party (PKK), deemed a terrorist group by the United States, European Union and Turkey, but the PKK has focused its campaign largely on domestic targets.
Turkey has led calls for international intervention in neighboring Syria and is hosting hundreds of NATO soldiers from the United States, Germany and the Netherlands who are operating a Patriot missile defense system along its border with Syria, hundreds of kilometers away from the capital.
The U.S. Patriots were expected to go active in the coming days.
The most serious attacks of this kind in Turkey occurred in November 2003, when car bombs shattered two synagogues, killing 30 people and wounding 146. Authorities said the attack bore the hallmarks of al Qaeda.
Part of the HSBC Bank headquarters was destroyed and the British consulate was damaged in two more explosions that killed a further 32 people a week later.