There was no immediate claim of responsibility for the attack, but Hakim blamed Taliban fighters.
General Abdul Raziq, border police chief for Zabul and Kandahar provinces, speculated that his forces may have been the intended target of the blast because as there is a police post on the same road.
There has been an upsurge in attacks by Taliban across Afghanistan as US-led and Nato forces have launched offensives aimed at seizing areas of southern Helmand province from Taliban control.
July has already become the deadliest the month for foreign troops in Afghanistan since the Taliban were forced from power in 2001.
The death of the British soldier on Thursday near Gereshk, the main industrial city in Helmand, took the toll for foreigntroops in Afghanistan in July to at least 47.
The previous monthly highs of 46 were set in June and August of 2008.
In less than seven months of 2009, 204 foreign troops have died in Afghanistan, compared with 294 in the whole of 2008, 232 in 2007 and 191 in 2006, the independent icasualties.org website, which calculates military losses in Afghanistan and Iraq, said.
Admiral Michael Mullen, the chairman of the US Joint Chiefs of Staff, has warned that Taliban fighters have become more dangerous and the US and NAto-led forces face
a crucial 18-month battle to help stabilise Afghanistan.
"The Taliban has got much better. They are much more violent, they are much more organised and so there's going to be fighting that is associated with it," he told Britain's BBC network.