A suicide bomber on a bicycle detonated explosives in central Kabul on Sunday, injuring six people, two days before a key international conference in the capital, a government official told AFP.
The bomb blast shook residential buildings in the capital, a witness said. An AFP photographer on the scene said the road was littered with body parts and a number of parked vehicles were badly damaged.
"It was a suicide attacker on a bicycle, he was trying to get to a specific area but because of high security the bomber was forced to detonate on a street where there is little activity," said Zemarai Bashary, spokesman for the interior ministry.
A reporter with Tolo television quoted witnesses saying the attacker had attempted to target a convoy of international forces, but this could not be immediately confirmed.
One witness described hearing the blast and immediately being shrouded in dust and smoke.
"It was heavy, it shattered the windows of buildings on both sides of the road," said Jawid Wardak. "I saw four or five people wounded. They were taken to hospital in civilian vehicles."
Kabul is set to host a major gathering of its international partners -- including about 40 foreign ministers -- on Tuesday, where the government will lay out is plan for the future.
Security forces have thrown a ring of steel around the city to head off any Taliban attacks, with police stationed every few metres (yards) along key streets and thousands of extra police officers on duty, authorities said.
Up to 70 international representatives, including US Secretary of State Hillary Clinton -- are due to attend the conference, to be co-chaired by President Hamid Karzai and UN Secretary-General Ban Ki-moon.
The conference is being billed as a bid by the Afghan government to start a process of transition from dependence on Western backers to running the country alone and responsibly.
Following Taliban attacks on a major domestic conference, or peace jirga, in early June -- which led to the dismissal of the interior minister and the head of the intelligence agency -- authorities said they were taking no chances.
At least two rockets were fired at the jirga as Karzai delivered his opening address. A suicide cell was later cornered and killed by security forces.
Zemarai Bashary, interior ministry spokesman, said on Saturday: "Security is on track."
"We're determined to provide good security for the Kabul Conference."
He said "thousands" of police officers, soldiers and intelligence agents had been deployed "in vulnerable areas" from the city centre to remote villages around the capital to thwart any Taliban attack plans.
"We hope to have a good day."
NATO's civilian representative in Afghanistan, Mark Sedwill, earlier told reporters that militant attacks on the conference could not be ruled out.
"We have to prepare ourselves for the fact that the insurgents will try to disrupt it," Sedwill said late Saturday, adding: "No one can offer 100 percent guarantees."
Extra police checkpoints have been established at the capital's entrances to screen all vehicles entering the city, Bashary said.
Kabul's airport is expected to be closed on Monday and Tuesday -- perhaps longer -- with most conference delegates arriving by private or government jet for just one day, a NATO official said, speaking on condition of anonymity.
Many staff with non-governmental organisations and embassies would spend the week in lockdown amid a raised security threat, charity workers, diplomats and others said.
NATO's International Security Assistance Force (ISAF) said Saturday its troops, working with Afghan counterparts, had captured a Taliban activist implicated "in coordinating an attack" on the conference.
Two other "facilitators" were captured earlier in the week, it said.
NATO and the United States have almost 150,000 troops in Afghanistan, battling an Afghan insurgency now almost nine years old and intensifying as the foreign forces take the fight to the increasingly bold insurgents.
In London, the Independent on Sunday newspaper said delegates to this week's conference would agree to hand over control of security in the country to Afghan forces by 2014.
Citing a leaked communique, it reported a phased transition beginning this year, along with pledges that foreign powers would continue to train, equip and finance Afghan security forces after that date.