The charge against Pakistan by Sibghatullah Mujaddedi, a former president of the country, who is now leading a reconciliation programme with the Taliban, is the latest round in bitter feud between the two countries over insurgent attacks in Afghanistan.
President Hamid Karzai has claimed that senior Taliban figures, including the former head Mullah Mohammed Omar, are living in Pakistan and using the country as a base to infiltrate fighters across the border. His officials accuse the Pakistani intelligence serevice, ISI, of recruiting and training suicide bombers.
Pakistan's foreign ministry spokeswoman, Tasnim Aslam, dismissed Mr Mujaddedi's allegations as baseless. President Pervez Musharraf has said that President Karzai is "oblivious" to conspiracy against Pakistan by the Afghan defence and foreign ministries. He has also insisted that a dossier presented to him by President Karzai detailing the Taliban presence in Pakistan was out of date.
Yesterday's suicide attack took place in the heart of the Afghan capital while Mr Mujaddedi was on the way to his office. The dead included the two bombers and two members of the public including a 12-year-old girl. Dismembered bodies could be seen lying on the blood- stained road in the aftermath of the explosion along with twisted wreckage of two cars.
Mr Mujaddedi, who suffered eye injuries, claimed that he was the target of a plot by the Pakistani secret service. "We have got information that ISI of Pakistan has launched a plan to kill me," he said.
Appearing at a news conference with his hands heavily wrapped in bandages, the result of burns he suffered when he raised them to protect his face, Mr Mujaddedi said: "The explosion was very strong. For a while I couldn't see anything. I was in the front seat of my car. I saw a big fire came toward me."
President Karzai condemned the bombing as "an attack on the voice of Afghanistan and clerics of Afghanistan." He did not directly apportion blame, but said he had received information two months ago of a plot to "attack important personalities in Afghanistan". Mr Mujaddedi, an Islamic cleric, served as president for two months in 1992 after mujahedin factions took control of Afghanistan after the pullout of Russian forces.
He was appointed chairman of the upper-house of the national assembly, Meshrano Jirga, by President Karzai in 2005. Mr Mujaddedi heads the commission responsible for reconciliation efforts with the Taliban and the government claims that several hundred of the former Islamist fighters have ended hostilities since negotiations began.
Afghan officials maintain that the ISI, which helped create the Taliban in the 1990s, wants to sabotage the reconciliation process.
The bomb attack on the Americans happened in the Pech Valley in Kunar province. The four Americans who died were travelling in a convoy of six when a remotely controlled mine exploded under their armoured vehicle.
The attack raised the total death toll of US personnel in the Afghan conflict to 220. About a hundred have died in the past 12 months in a fresh campaign by a resurgent Taliban.