An Afghan intelligence official said on Friday about 60 of these bombs have been made in Pakistan and smuggled into Afghanistan by Islamic fighters belonging to the Hezb-i Islami movement, the official said, requesting anonymity.
The bombs, which carry between 0.5 and 2 kilograms of explosives, can be detonated by remote control and are attached by a magnet to the body of a vehicle.
The sophisticated devices would be used to target vehicles belonging to the NATO-led International Security Assistance Force charged with maintaining security in Kabul, he added.
They could also be used against cars belonging to the United Nations and foreign organisations by militants loyal to the Hezb-i Islami movement of former Afghan premier, fugitive Gulbuddin Hekmatyar, or other insurgents, he warned.
ISAF has informed its personnel of the dangers of these devices and taken the necessary measures, the source said. ISAF spokesman Commander Chris Henderson refused to comment on the devices, citing security reasons.
In January 2003, the United Nations informed its workers of a similar alert.
On January 27 and 28 of this year, two suicide attacks were carried out against ISAF soldiers, killing a Canadian soldier, a British soldier, an Afghan civilian and the bombers themselves. More than 10 others were injured in the blasts.
Afghan intelligence services also believe bombers are planning attacks in which fake UN vehicles packed with explosives will be detonated.
Four, four-wheel-drive vehicles painted in white and marked as UN cars, and even equipped with radios, have been prepared by groups wanting to stage a major attack in Kabul, but have not yet reached the city, the source said.
Checkpoints into the city have been told to watch out for these cars which are expected to try to enter Kabul from the west, he said.