A suicide bomber killed 16 civilians and three foreign troops on patrol in the provincial Afghan capital of Khost on Wednesday, the U.S. embassy and police said, in one of the deadliest attacks for weeks as the Western military prepares to pull out.

Khost, a volatile eastern region bordering Pakistan, is an operations hub for the Taliban-linked Haqqani group which the United States says has been behind a string of bombings in Kabul as well as attacks on foreign forces in the countryside.

Earlier this month Defense Secretary Leon Panetta vowed to renew efforts to fight the Haqqani who he blamed for an attack on a NATO base on June 1 in Khost that killed one U.S. soldier, a U.S. contractor, an Afghan civilian and 14 insurgents.

Three members of the NATO-led International Security Assistance Force and an Afghan interpreter were killed in Wedensday's bombing, U.S. embassy spokesman Gavin Sundwall said in a statement.

The bomber was riding a motorbike and detonated his explosives near a mosque in a crowded part of Khost city, Sardar Mohammad Zazai, police chief of the province said. He said 16 civilians were killed.

At least 30 people were wounded, including women and children, local officials said.

A witness said that NATO and Afghan troops were using biometric data to screen residents of the provincial capital when the bomber struck.

"Then suddenly there was an explosion. I saw many killed and wounded including Afghan police and foreign troops. We carried some of them to the hospital," said Gul Mohammad, who has a shop near the scene of the blast.

Afghanistan is in the third phase of a five-stage transition in which the centres of provincial capitals, including in the violence-plagued south and east, will be handed to Afghan forces before the withdrawal of most foreign combat troops in 2014.