A far-left suicide bomber has killed a Turkish security guard at the US embassy in Ankara, officials said, blowing open an entrance and sending debris flying through the air.
The attacker detonated explosives strapped to his body after entering an embassy gatehouse. The blast could be heard a mile away. A lower leg and other human remains lay on the street.
Interior Minister Muammer Guler said the bomber was a member of an illegal far-left group. The White House said the suicide attack was an "act of terror", but the motivation was unclear.
Islamist radicals, extreme left-wing groups, ultra-nationalists and Kurdish militants have all carried out attacks in Turkey in the past. There was no claim of responsibility.
"The suicide bomber was ripped apart and one or two citizens from the special security team passed away," said Prime Minister Tayyip Erdogan, who was attending a ceremony in Istanbul when the blast happened.
"This event shows that we need to fight together everywhere in the world against these terrorist elements," he said.
Turkish media reports identified the bomber as Ecevit Sanli, a member of the Revolutionary People's Liberation Party-Front (DHKP-C) leftist group, who was involved in attacks on a police station and a military staff college in Istanbul in 1997.
The DHKP-C opposes what it sees 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 and has been one of the leading advocates of foreign intervention to end the conflict in neighboring Syria.
Around 400 US soldiers have arrived in Turkey over the past few weeks to operate Patriot anti-missile batteries meant to defend against any spillover of Syria's civil war, part of a NATO deployment due to be fully operational in the coming days.
US Ambassador Francis Ricciardone emerged through the main gate of the embassy, 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're very sad of course that we lost one of our Turkish guards at the gate," Ricciardone said, describing the victim as a "hero" and thanking Turkish authorities for a prompt response.
A US national security source said US officials believed the incident was a suicide bombing but said security measures had worked properly, in that the attacker was not able to get past the outer perimeter of the compound and neither the embassy buildings were damaged, nor were US personnel injured.
It was the second attack on a US mission in four months. On September 11, 2012, US Ambassador Christopher Stevens and three American personnel were killed in an attack on the US Consulate in Benghazi, Libya.
The attack in Benghazi, blamed on al Qaeda-affiliated militants, sparked a political furore in Washington over accusations that US missions were not adequately safeguarded.
A well-known Turkish journalist, Didem Tuncay, who was on her way in to the embassy to meet Ricciardone when the attack took place, was in a critical condition in hospital.
"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 metres away from the blast.
OPPOSED TO US INFLUENCE
The DHKP-C, deemed a terrorist organisation by both the United States and Turkey, has been blamed for suicide attacks in the past, including one in 2001 that killed two police officers and a tourist in Istanbul's central Taksim Square.
Guler said the bomber could have been from the DHKP-C which has carried out a series of deadly attacks on police stations in the last six months, or a similar group.
The attack may have come in retaliation for an operation against the DHKP-C last month in which Turkish police detained 85 people. A court subsequently remanded 38 of them in custody over links to the group.
The US consulate in Istanbul warned its citizens to be vigilant and to avoid large gatherings, while the British mission in Istanbul called on British businesses to tighten security after what it called a "suspected terrorist attack".
In 2008, Turkish gunmen with suspected links to al Qaeda, opened fire on the US consulate in Istanbul, killing three Turkish policemen. The gunmen died in the subsequent firefight.
The most serious bombings in Turkey occurred in November 2003, when car bombs shattered two synagogues, killing 30 people and wounding 146. Part of the HSBC Bank headquarters was destroyed and the British consulate was damaged in two more explosions that killed 32 people less than a week later. Authorities said those attacks bore the hallmarks of al Qaeda.