The bomber approached a group of Afghan men who were waiting outside the base in the city of Khost, in Khost province, before triggering an explosive strapped to his chest, said Jamal Arsalah, the province's governor.

Maj Matt Hackathorn, a US military spokesman, confirmed the blast, but said that there were no US military casualties known so far.

Jamal said the men were daily workers, lining up to get into the base at the time of the blast.

Suicide attacks have increased as Taliban militants intensify their insurgency against Afghan government forces and foreign troops. According to US military figures, there were 139 suicide attacks during 2006, up from 27 in 2005.

In 2006, Taliban carried out a record number of attacks suicide and otherwise and some 4,000 people died in the insurgency-related violence, according to a tally by The Associated Press based on reports from Afghan, Nato and coalition officials.

Khost, where Tuesday's attack was carried out, is a former al-Qaeda stronghold on the mountainous Pakistani border that has been a focus of militant activity.