The blast occurred around 8:40 am (0410 GMT) when an explosives-laden vehicle drove into the convoy that was travelling in eastern Kabul, according to interior ministry spokesman Nasrat Rahimi.

"Unfortunately in the attack, four of our countrymen were martyred and three others wounded -- they were passers-by", Rahimi said.

"This is our initial information and the figures will likely change."

There was no immediate word on foreign casualties.

Resolute Support, the US-led NATO mission in Afghanistan, did not immediately comment.

The attack came one day after an Islamic State suicide bomber blew himself up outside an Afghan army military academy, killing at least six people.

It also followed a meeting between the Taliban and Afghan opposition figures in Moscow this week that yielded no tangible results.

Afghan President Ashraf Ghani had proposed a nationwide ceasefire at the start of the holy month of Ramadan, but the Taliban rejected the offer and violence continues apace across the country.