A suicide bomber has rammed his car into an Afghan army vehicle providing security for a compound where Afghanistan's political and tribal elites are due to gather next week to debate a security pact with the United States.

Saturday's attack took place just hours after President Hamid Karzai called on the Taliban to take part in the Loya Jirga assembly that convenes on Thursday to decide whether to allow some U.S. troops to stay in Afghanistan after 2014.

At least six people were killed and 22 wounded in the blast, said Sediq Sediqqi, an Interior Ministry spokesman. He said the casualties were a mix of soldiers and civilians.

The blast occurred shortly after 3 p.m., fewer than 100 metres from a huge tent where more than 2,000 prominent Afghans are due to gather, a Reuters witness said.

No one immediately claimed responsibility for the attack.

One man fleeing the bombing, Mohammad Amin, looked dazed as he described to Reuters seeing a white Totoya Corolla vehicle speed towards a police checkpoint and then explode.

Covered in blood and dust, he said he was standing across the street behind his parked car when the blast occurred.

"Thank God my car protected me because it was so close. My ears are still ringing," Amin said.

A Reuters reporter saw at least six wounded people, as well as a large unchecked fire and numerous smashed cars.

A Kabul police spokesman, Hashmat Stanekzai, said earlier there were at least 15 dead and wounded as a result of the attack, but could not immediately provide more detail.

U.S., Afghan draft agreement completed

The explosion came just hours after President Hamid Karzai announced that U.S. and Afghan negotiators had finished a draft to be presented to the Loya Jirga, whom Kabul says must approve the document before Afghanistan signs it.

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 will likely refuse to sign the agreement, Karzai said. And if the Loya Jirga does approve it, the agreement still will require a final nod from parliament.

U.S. officials refused to comment on what they described 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 final contents of a 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. After a lengthy meeting with Karzai, the two announced that an agreement had been reached in principle on the major elements of the pact.

The sweeping document incorporates the usual Status of Forces Protection Agreement, which the United States 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 can be prosecuted for criminal offences in Afghanistan.

"Because this is an ongoing diplomatic discussion, we're declining to comment on the state of the text or the process that got us to this point," said Robert Hilton, U.S. Embassy spokesman in Kabul.

"The draft is completed," said Karzai, adding however that "there are still a few special points still being discussed ... There are still some differences."

He refused to elaborate on the differences or whether they could be deal breakers with Washington.

Security talks to focus on insurgents from Pakistan

But earlier, two senior U.S. officials told the AP that Afghanistan had sought specific security guarantees, particularly against cross-border incursions by insurgents from neighbouring 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 sensitivity of the issue.

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 they (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, where 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, which is to begin on Thursday, will involve about 3,000 elders, clerics, parliamentarians and influential figures. The debate is expected to last several days and attendees are expected to be deeply divided over signing the pact.

"The Loya Jirga will bring together all the people, who agree and disagree about the Bilateral Security Agreement. We want them to talk about the agreement in a free and fair environment without any pressure on them," Karzai said urging only that the attendants read the agreement carefully and in detail. He asked that they study each clause before coming to the Jirga.

"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," he said.

A majority negative vote from the Jirga will likely 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 are also expected to stay in Afghanistan to hunt down al-Qaeda and carry out counter-terrorism activities.

With files from The Associated Press