Had it not been for the alertness of two Afghan policemen, the damage caused by the suicide bomber who struck at the Indian embassy in Kabul on Thursday could have been much worse.

Constables Alam Khan and Mohammad Usman of Afghan National Police died in the blast but not before preventing the vehicle carrying over 100 kgs of explosives from gaining access to the gate of the Indian embassy, revealed official sources.

Khan and Usman apparently got alerted when they spotted a vehicle moving near the embassy in suspicious manner during their routine patrolling, they said.

They stopped the vehicle at the barricade and did not allow it to pass, prompting the suicide bomber to detonate the explosives there itself, they said.

The sources said the quantity of explosives used in the attack was much more than that was used in July 2008 blast.

During that attack, about 100 kgs of RDX had been used, killing 60 people, including four Indians, and injuring over 100 others.

In Thursday's attack, 17 people were killed and over 80 injured.

Foreign Secretary Nirupama Rao, who landed in Kabul on Friday to take stock of the situation, hailed the bravery and courage of the two policemen.