Up to 50 people, one UN staffer among them, were wounded in the bombing on the busy Kabul-Jalalabad Highway, frequently used by American forces, the Interior Ministry said.

"I am shocked and greatly saddened to have learned that two of my staff members were among those killed in todays suicide bombing on Jalalabad Road," said the UN secretary-general's special representative in Afghanistan.

Kai Eide added a third UN worker wounded in the explosion was currently being treated for his injuries. All the three are Afghan citizens. "At this time UNAMA is in the process of contacting the families of our colleagues to inform them of this tragedy. I condemn completely those responsible."

In a statement, the ministry said 18 vehicles were damaged by the blast that took place on the highway two days ahead of the presidential and provincial council elections -- reinforcing widespread security concerns.

NATO said International Security Assistance Force (ISAF) soldiers among the casualties of the vehicle-borne improvised explosive device. Afghan and foreign forces were assessing casualties and damage, the Western military alliance said.

"Reports indicate both Afghan civilians and ISAF service members were killed and wounded in the blast as the vehicle exploded near an ISAF convoy and other civilian vehicles," it added.

Col. Wayne Shanks, an ISAF spokesman, said: "This is yet another indiscriminate attack on both the people of Afghanistan and our forces who are working with our Afghan partners to protect them."

Shanks said: "I want to express our sincere condolences to the families for their loss, but these words are not adequate to capture the loss of any service member or civilian and the grief their families must be experiencing."

A shopkeeper in the area earlier told Pajhwok Afghan News at least two civilians perished in the massive bombing. "A heavy explosion shook my shop and there was a cloud of dust all around," Abdul Aziz said.

His son was among those wounded in the daring suicide assault, the second in Kabul in four days, Aziz added. He saw the wounded shifted to hospital from the scene.

Kabul Crime Branch Chief Syed Abdul Ghaffar Syedzada said the suicide bomber used a Toyota Corolla car in the attack. He explained the dead and injured were all civilians. However, he stopped short of putting a figure on the casualties.

One vehicle owned by the United Nations, caught fire in the wake of the explosion, one eyewitness claimed. A UNAMA spokesman confirmed three Afghans working for the world body were injured in the incident.

Hours earlier, suspected Taliban militants fired two rockets into Kabul, with one landing near the Presidential Palace. The rockets fell between the Ministry of Defence and the Presidential Palace at 7am.

But Defence Ministry spokesman Gen. Zahir Azimi, when approached for comments by Pajhwok Afghan News, said no rocket had struck the ministry.

As usual, Taliban moved swiftly to assert responsibility for the attack that came two days ahead of the presidential and provincial council polls. Militant spokesman Zabihullah Mujahid said the target was the Presidential Palace.

mud/pr