The attack was the first major blast in the Afghan capital since Taliban militants launched a suicide attack on January 17 against a popular restaurant killing 21 people, including 13 foreigners.

"The bomber was on foot when he detonated himself next to a bus carrying Ministry of Defence staff to work," interior ministry spokesman Seddiq Seddiqi told AFP.

"Initial reports from the police say that three people in the bus were killed and one female bystander."

At least nine people were wounded in the attack in the southeast of the city.

A Taliban spokesman using a recognised Twitter account claimed responsibility for the blast.

NATO combat forces are withdrawing from Afghanistan this year after more than a decade of fighting the Taliban, but negotiations have stalled over a deal to allow some US and NATO troops to stay after 2014.

President Hamid Karzai on Saturday signalled that the bilateral security agreement (BSA) was close to collapse after he made a surprise decision last year not to promptly sign an agreed text.

Washington has become increasingly frustrated by Karzai's manoeuvreing over the deal, with some US politicians pushing for a complete US pull-out by December.

"Afghanistan will absolutely not accept or sign anything under pressure," Karzai told a press conference.

"If they want to leave, then they can go and we will continue our lives... Our main condition is the practical start of peace process.

"The start of a peace process would mean that no foreigners can benefit from the continuation of war."

The US had earlier wanted for the BSA to be signed by the end of October so that the NATO military coalition could schedule the withdrawal of its troops.

About 58,000 NATO-led combat troops who are still in Afghanistan are due to leave by the end of 2014.

Washington is proposing that 5,000 to 10,000 US soldiers be deployed from 2015 to train and assist Afghan security forces in their battle against the Taliban militants.

Afghanistan's fledgling security forces face a difficult year in 2014 as insurgents attempt to disrupt elections on April 5 that will choose a successor to President Karzai, and as NATO's combat mission winds down.

Signing the BSA is also a precondition for the delivery of billions of dollars in Western aid for Afghanistan, but Karzai dismissed suggestions that the aid was essential to future development.