Gen. Zahir Azimi, a spokesman for the Ministry of Defense, said a car packed with explosives rammed an army vehicle at about 3 p.m. Siddiq Siddiqi, a spokesman for the Ministry of Interior, said the suicide bomber killed at least six people and wounded 22 others.
Afghan journalists at the scene reported as many as 15 people had been killed and dozens wounded. More than a dozen vehicles were also set on fire.
There was no immediate claim of responsibility for the attack, the first major incident in Kabul in several months.
The bombing occurred just hours after Karzai announced at a news conference at the presidential palace that he and U.S. negotiators have completed work on a security agreement that will be considered during the five days of talks.
The event is called a loya jirga, a forum that Afghans have used for centuries to discuss vital national issues.
Karzai declined to discuss the specifics of the agreement but said he hopes the loya jirga results in a broad consensus that allows several thousand U.S. troops to remain in Afghanistan in training and advisory roles.
Robert Hilton, a spokesman for the U.S. Embassy in Kabul, also declined to comment, saying the agreement was a matter of “ongoing diplomatic discussion.”
The Obama administration has been eager to sign a long-term security agreement, but there has been considerable resistance in Afghanistan over whether U.S. troops would be granted immunity from prosecution for crimes committed inside the country.
On Sunday, several hundred Afghan politicians and tribal leaders met in Kabul to preemptively denounce the upcoming loya jirga, saying they oppose a continued U.S. presence, regardless of the outcome of the gathering.
“We will consider the presence of a single foreign soldier in Afghanistan as an invasion,” said one participant, Ahmad Shah Ahmadzai, who served as Afghan prime minister from 1992 to 1996. “Signing this deal will mean an escalation of the fighting and will further destroy Afghanistan.”
In his news conference Saturday, Karzai pushed back at his critics, saying the final decision on the proposal will be left up to politicians, lawyers, tribal elders and scholars from all 34 provinces gathering this week.
“The jirga will scrutinize all articles of the BSA and then decide based on Afghanistan’s interests,” Karzai said, referring to the bilateral security agreement.
He added that representatives of the Taliban and Hezb-i-Islami, both of which are seeking to drive out all foreign troops from the country, also have been invited to attend the loya jirga.
So far, they have declined, he said.
Even before Saturday’s bombing, security in Kabul was tightened amid concerns of violence targeting the loya jirga, the first such event in two years. U.S. military officials are also bracing for more attacks than usual this winter because of the ongoing debate and pending pullout of most NATO troops.
“Insurgent groups will be trying their best to disrupt the jirga,” said Javid Kohistani, a Kabul-based security and political analyst. “There is no doubt insurgents have a plan to conduct similar attacks near the loya jirga and other parts of the city” this week.