At least six people have been killed which include a police officer and more than 10 injured in a bomb attack in the Afghan capital, Kabul.

The blast occurred less than 100m from a compound where tribal elders are expected to gather next week to discuss a security pact with the US.

Police say the blast was caused by a suicide bomber. No group has said it carried out the attack.

 The attacker crashed a coach filled with explosives into an Afghan army vehicle when soldiers identified the vehicle as a threat and opened fire. Police said.

The security pact governs the status of US military personnel staying in Afghanistan beyond 2014.

The security agreement must be approved by the Loya Jirga and parliament before it can come into force.

Among other issues, the pact covers the key issue of whether US military personnel will enjoy immunity from prosecution in Afghanistan.

President Hamid Karzai has also called on the Taliban, who are fighting an insurgency against the foreign presence in Afghanistan, to attend the Loya Jirga.

"We invite them, please come to this national Jirga of Afghanistan, raise your voice, raise your objection." He said.

 Between 5,000 and 10,000 US military personnel are due to remain in the country beyond the end of 2014, when most of the Nato-led international force is due to have left Afghanistan.

The US is expected to focus on training and mentoring Afghan security forces as well as combating al-Qaeda.