The blast -- similar to scores blamed on extremists behind a bloody insurgency in Afghanistan -- hit the vehicles after they left the southeastern city of Khost, the police chief of Mando Zayi district said.

"They were on their way to their work site when their vehicles were struck by an explosion and three people, including a foreign engineer -- an Indian engineer – were wounded," said the official, Mohammad Jamal. The identity of the Indian engineer was not known immediately.

It was not clear if the blast was a landmine or remote-controlled bomb, Jamal said. Two vehicles were damaged in the explosion, he said.

India runs several important reconstruction projects in Afghanistan as part of international efforts to help the country rebuild after nearly three decades of war.

The group hit in today's blast were working on a road between Khost and the neighbouring town of Gardez. President Hamid Karzai last month opened a 217-kilometre road built by India to connect southern Afghanistan to the border with Iran that is intended to give the landlocked country access to Iranian ports.