Two men, driving a car filled with powerful explosives were stopped because of a traffic accident, the official said, adding that the plotters, probably suicide-bombers, were intending to blow up the government’s office, or the American embassy or garrison used by international forces.

"If exploded, the explosion would have killed a lt of people in a radius of 500 metres, as it was a very powerful and professional work", said Mr. Amrollah Saleh, of the Afghanistan’s Security Organisation.

He said according to information obtained, Hezb Eslam (Islamic Party) is working closely with both al-Qa’eda and the ousted Taleban to destabilise Karzai’s government.

But he did not say who is behind the destabilisation effort.

Pakistan newspapers accused the neighbouring Islamic Republic of Iran to disseminate propaganda and subversive material among the majority Poshtoon population, aimed at stirring inter-ethnic fightings.

Foreign intelligence information indicated the alleged terrorist, who was captured by Afghan authorities on Monday after a car chase through Kabul, was a foreigner and a member of the al-Qaida terror group, he added.

"He was on a suicide mission", confirmed Mr. Amanollah Barakzai, another senior intelligence officer. But he said Afghan authorities were not sure whether his target was the American embassy, other foreign installations or the Afghan leadership, including President Hamid Karzai, whose palace offices are about a half-mile from the accident scene.

One unidentified Afghan in the car also was arrested, and another Afghan escaped, said General Din Mohammad Jor’at, chief of Interior Ministry police. The investigation's focus has been on the foreigner, suggesting the Afghans were believed to be playing secondary roles.

The government sent additional troops into the capital's streets late Monday to guard against new attacks.

The alleged car-bombing attempt was the latest development feeding concern about security in the capital Kabol, in the wake of the killing of Vice President Abdol Qadir three weeks ago.

Investigator found 500 or 600 kilograms of TNT and C-4 explosives, rigged with an electrical detonating system, Afghan intelligence sources said.

The government TV report late Monday said the suspect was injured in the car accident and was taken to the intelligence service's offices for interrogation.