A security guard has been killed after a suicide bomber blew himself up outside the US embassy in the Turkish capital, Ankara, the provincial governor says.

"There were two dead in the suicide bombing, a Turkish security guard and the bomber himself," Alaattin Yuksel told reporters, adding that a woman was also wounded in Friday's blast in the Cankaya neighbourhood.

The US ambassador to Turkey confirmed that one of the embassy's Turkish staff was killed.

"We are very grateful to the Turkish state and the police for the very prompt response," said Francis Ricciardone. "We are very sad of course that we lost one of our Turkish guards at the gate."

Recep Tayyip Erdogan, the Turkish prime minister, said the bomber was a member of the Revolutionary People's Liberation Party-Front (DHKP-C), a far-left group which is virulently anti-American and anti-NATO.

Turkish media identified the bomber as Ecevit Sanli, who was involved in attacks on a police station and a military staff college in Istanbul in 1997.

The US is calling the attack an "act of terror," but said the motivation was unclear. US officials said that the DHKP-C was the main suspect, but did not exclude other possibilities.

Investigations under way

Television footage showed a door blown out and masonry from the wall around it scattered in front of the entrance, although there did not appear to be any more significant structural damage. The explosion was heard more than a kilometre away.

Smoke rose from a side entrance to the embassy after the doorway was damaged and debris sent flying into the street. The bomb appeared to have exploded near a security checkpoint at the entrance of the visa section of the embassy.

"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.

Police cordoned off the street where many other state institutions and embassies, including those of Germany and France, are also located. A police helicopter hovered in the air and armed US Marines patrolled the embassy roof following the attack.

Victoria Nuland, a US state department spokesperson, said that investigations were under way to find those responsible.

"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 said in a statement.

In a statement made after the bombing, Muammer Guler , the Turkish interior minister, said that the bomber was about 30 years old, and was a member of a far-left group.

"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."

In July 2008, three gunmen and three Turkish policemen were killed in an attack outside the well-fortified US consulate in Istanbul.