* Afghan elders due to debate future of US-Afghan pact
* Blast outside venue's tent, casualties unclear
* Witness saw vehicle speeding towards venue before blast
An apparent suicide attack struck Kabul on Saturday next to the venue where Afghanistan's political and tribal elites will debate a vital security pact with the United States, in a major security breach just days ahead of the crucial assembly.
A large blast was heard soon after 3pm (1030 GMT) in Kabul's west, a Reuters witness at the scene said. A Kabul police spokesman, Hashmat Stanekzai, confirmed the blast and said there were casualties but could not immediately confirm a number.
The blast occurred less than a hundred metres from a huge tent where more than 2,000 prominent Afghans will gather on Thursday to debate a security agreement with the United States once the NATO mission ends next year.
A man fleeing the bombing, Mohammad Amin, who was dazed and covered in blood and dust, described seeing a White Corolla vehicle speed towards a police checkpoint and explode.
Amin said he was standing across the street from the explosion, 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 told Reuters.
A Reuters reporter saw at least six wounded people, as well as a large unchecked fire and numerous smashed cars.
The attack occurred on a road about 80 metres (240 feet) from the entrance to the compound of the Loya Jirga. A loya jirga is a traditional Afghan meeting convened to debate matters of national importance and includes thousands of tribal elders, politicians and other elites.
A loya jirga is due to begin in five days at the tent, in order to debate the approval of a security pact with the United States.
Afghanistan and the United States have not yet agreed on several issues in a bilateral security pact, raising the prospect that Washington may yet pull out all of its troops next year unless differences are ironed out.
Two years ago, the United States ended its military mission in Iraq with a similar "zero option" outcome after the failure of talks with Baghdad.
For almost a year, Washington and Kabul have been seeking to conclude a Bilateral Security Agreement (BSA) that will help determine how many U.S. soldiers and bases remain in Afghanistan after most foreign combat troops exit by the end of next year.