Authorities said 22 people were wounded in the powerful blast, which mangled a dozen cars and destroyed shops nearby. Ambulances raced away with the wounded.
The explosion came just hours after President Hamid Karzai announced that U.S. and Afghan negotiators had finished a draft deal to be presented to the Loya Jirga, whom Kabul says must approve the document before Afghanistan signs it.
The explosive-laden vehicle rammed into an armored vehicle posted about 200 meters (yards) from the giant tent where the Loya Jirga is to be held, Defense Ministry spokesman Gen. Mohammed Zahir Azimi said.
No group immediately claimed the attack, though blame is likely to fall on the Taliban, who have adamantly opposed the presence of any foreign soldiers in Afghanistan.
Karzai has called 3,000 elders, clerics, parliamentarians and other influential figures to debate the Bilateral Security Agreement, which would allow U.S. troops to remain in Afghanistan after the final withdrawal of international combat troops at the end of 2014.
Without approval of the Loya Jirga, Afghanistan likely will refuse to sign the agreement, Karzai said. If the Loya Jirga does approve it, the agreement still requires final approval from parliament.
U.S. officials refused to comment on the draft, describing the effort as an ongoing diplomatic process. Karzai provided few details regarding how and when the draft was finalized, but said there still remain "differences" between Washington and Kabul on the deal.
Negotiations have been protracted and often acrimonious. In the end it took a surprise visit to Afghanistan in October by U.S. Secretary of State John Kerry to produce the outlines of a deal.
The sweeping document incorporates the usual Status of Forces Protection Agreement, which the U.S. signs with every country where its troops are stationed, along with a wide range of other clauses. It covers everything from customs duties on goods the U.S. imports for its troops and development projects to the question of whether a U.S. service member could be prosecuted for criminal offences in an Afghan court.
Earlier, two senior U.S. officials told The Associated Press that Afghanistan had sought specific security guarantees, particularly against cross-border incursions by insurgents from neighboring Pakistan. Washington is cautious about any commitments that could lead to a conflict with Pakistan. The officials spoke on condition of anonymity because of the deal was still being negotiated.
Karzai described a laborious negotiation process that sometimes came down to fine details of phrasing.
"There was one word that we didn't want in the agreement but (the U.S.) wanted and in the end they agreed to not use that word," he said, without identifying the offending word.
Karzai did not say what the draft said regarding U.S. service members' immunity from prosecution. This key American demand has been a sore point in Afghanistan. Many are still angry over incidents including the February 2012 accidental burning of hundreds of copies of the Islamic holy book, the Quran, a March 2012 shooting spree by a U.S. soldier in southern Afghanistan that killed 16 people, and unintended civilian deaths from U.S. bombs.
The Loya Jirga is scheduled begin Thursday. The debate is expected to last several days and attendees are likely to be deeply divided over signing the pact.
"They should think about the prosperity and stability of today and tomorrow in Afghanistan. And whatever decision they are making they should think about the future of Afghanistan," Karzai said.
A "no" vote from the Jirga likely will scuttle the agreement and leave Afghanistan without any U.S. forces after the end of 2014. With the agreement, the residual force of about 10,000 that is expected to remain behind would mostly train and mentor Afghanistan's National Security Force. A small group of U.S. Special Forces also are expected to stay in Afghanistan to hunt down al-Qaida fighters and carry out counter-terrorism activities.