KABUL -- International peacekeepers in Afghanistan said on Monday they were investigating an explosion in the capital Kabul which killed one Afghan and injured three people, including a British soldier.

An explosive device went off on Sunday afternoon in southwest Kabul as two vehicles belonging to the International Security Assistance Force (ISAF) drove past.

"At this moment we are not certain if this is a terrorist attack," said ISAF spokesman Major James Kelly.

"A detailed investigation is under way by ISAF Explosive Ordnance Device (EOD) teams and samples have been recovered for inspection and analysis," he told a news briefing.

Kelly said it was the first explosion which may have been aimed specifically at the 5,000-strong peacekeeping force in Kabul.

The British soldier was hit in the wrist by shrapnel, but was treated in Kabul and returned to his duties hours later.

Kelly said ISAF had not raised its security level after the blast.

The International Committee of the Red Cross (ICRC) in Afghanistan said on Monday it had stopped using a key supply route to Kabul for security reasons after a blast narrowly missed one of its vehicles close to the road.

Julian Harris, head of the ICRC's delegation in the eastern city of Jalalabad, said his Organization was in talks with local authorities about the explosion near the town of Sarobi, some 60km (40 miles) east of Kabul.

--- Series of Explosions --- There have been around five unexplained explosions in the capital in the last three weeks, one of them outside a UN guest house.

"There have been a number of explosions, most of which we have identified as being non-threatening," Kelly said.

ISAF sources said that the most likely theory for Sunday's blast was a deliberate explosion.

The Afghan who died was cycling past a wooden hand-cart, used to carry fruit and other merchandize, when the device went off. Police at the scene said it appeared the explosives were planted in the cart.

"The early indications are that it was an old shell," Kelly said.

Earlier on Sunday, four Afghans were killed in two separate incidents involving mines close to Bagram Air Base, the U.S. military headquarters just north of the capital.

One Afghan de-miner died when an anti-personnel mine went off. An ambulance belonging to a Danish de-mining organization went to collect the body, and just after it turned back for Kabul it hit an anti-tank mine, killing three Afghans in the vehicle and injuring at least 18.

A U.S. military statement said that 12 wounded Afghans were being treated at a Spanish hospital and six more were at the U.S. Army Hospital at Bagram. A U.S. spokeswoman had nothing further to add.

Millions of undetected mines are hidden across Afghanistan, the legacy of Soviet occupation, civil conflict and the Taleban's war against the Northern Alliance resistance.

The strategic air base at Bagram is particularly heavily mined.