A suicide car bomb has killed at least three civilians and the bomber himself in the southern Afghan city of Kandahar, officials say.
The Taleban has claimed responsiblity for Monday's suicide attacks
Kandahar Governor Asadullah Khalid said the attacker rammed his car into a convoy of US and Afghan forces.
This is the third Afghan suicide attack in as many days and the government says it suspects al-Qaeda involvement.
The Kandahar attack came as five Afghan soldiers were killed in a bomb blast in Paktika province, officials say.
Wednesday's explosion in Kandahar comes two days after twin suicide car bomb attacks in the capital, Kabul, which killed at least nine people.
At least four Afghan civilians were injured in the rush-hour attack on Wednesday, said a spokesman for Kandahar's governor.
The blast happened just before 0900 local time (0430 GMT), when a Toyota Corolla laden with explosives drove into an armoured Landcruiser of the US special forces, a senior security official told the BBC.
US military spokesman Lt Col Jerry O' Hara confirmed the bombing but denied reports that three US soldiers were injured in the attack.
An eyewitness, Ghulam Mohammed Haq, said he saw a car ram into a four-wheel-drive vehicle before exploding.
September 2005: 12 killed outside Afghan army base in Kabul
May 2005: 3 killed in Kabul internet cafe
June 2005: 20 killed in Kandahar mosque
October 2004: 3 killed in Kabul shopping centre
August 2004: 10 US security contractors killed in Kabul
"I saw some people being carried away wounded," he said.
The Taleban have said they carried out this week's attacks in the two cities.
The BBC's Andrew North in Kabul says the apparent co-ordination of the attacks is causing concern to the Afghan authorities.
Defence Minister Rahim Wardak underlined those fears on Wednesday when he repeated warnings he has made before that he thinks al-Qaeda is party responsible.
Our correspondent says there is some evidence for this. Since spring there have been reports that al-Qaeda supporters are channelling money and equipment to local Taleban commanders along the frontier.
But he says it is not yet clear whether these latest attacks represent a new campaign by the Taleban and other groups.
The latest violence comes as discussions are going on within Nato about how to fulfil commitments to expand the peace keeping force in Kabul to areas currently controlled by the US-led coalition force.
The US would like to start withdrawing some of its troops from the region.
Britain will lead the peace keeping force next year, but some other Nato countries have said they are unwilling to get involved in fighting insurgents.
More than 1,400 people have been killed in violence linked to militants in Afghanistan this year - the worst violence the country has seen since US-led forces ousted the Taleban in late 2001.
Most of the violence has been in the south and east of the country.