KABUL: A suicide bomber riding a motorcycle blew himself up in the heart of Afghanistan's southwestern town of Farah, killing six people and wounding 36 civilians, the provincial police chief said.

The attack occurred the day after Afghan President Hamid Karzai was sworn in for a second term, pledging to try to bring peace to the nation and take over security from foreign forces in five years.

The bomber struck in an area where heavy trucks were being loaded up with goods travelling from Farah to Herat, police said.

"A suicide bomber with a motorcycle carried out a suicide attack in the centre of Farah town in the area of Ada Herat," said Faqir Ahmad Askar, police chief of the southwestern province of Farah.

"As a result, six people were killed including a police officer. So far, 36 civilians were wounded," he told AFP.

Provincial governor Rohul Amin confirmed the incident and put the toll at five dead and 36 wounded, including 15 in serious condition.

"The bomber riding on a motocycle detonated himself at a main square near my working office in my home," he said, adding that the blast caused damage to some nearby buildings.

The suicide bombing a day after police said Afghan security forces backed by tribal militia killed six Taliban militants in the same province after the rebels beheaded two tribal leaders on the weekend.

The Taliban-led insurgency dogged Afghanistan is now at its deadliest in the eight years since US-led troops ousted their extreme Islamist regime in Kabul, slowly encroaching into previously peaceful parts of the north and west.

But Karzai used his inauguration speech in Kabul on Thursday to express hopes that Afghan troops would soon be taking the lead for security, allowing the more than 100,000 NATO and US troops to scale back.

"We are determined that within the next five years the Afghan forces are capable of taking the lead in ensuring security and stability across the country," he said.

British Prime Minister Gordon Brown has also proposal for a timetable for a gradual security handover from 2010, but US Defence Secretary Robert Gates said it was too soon to set a timeline.

"I think I would rather have those on the ground in Afghanistan make the judgment call about when a province or a district was ready to be turned over, rather than specific dates," Gates told reporters on Thursday.