KABUL - A powerful blast targeting an armored NATO convoy in Kabul killed at least eight people and wounded 27 on Wednesday, including three coalition soldiers, officials said days after the Taliban announced their spring offensive.

The attack, which came during morning rush hour on a busy road near the US embassy and NATO headquarters, killed mostly civilians, an Interior Ministry spokesman said without giving a breakdown.

NATO said three coalition soldiers had received "nonlife threatening wounds".

"(They) are in stable condition, and are currently being treated at coalition medical facilities," a spokesman for US Forces-Afghanistan said, without confirming their nationalities.

No group has yet claimed responsibility for the blast, which underscores the threat faced by international and Afghan forces against the resurgent Taliban as the US seeks to craft a new strategy in Afghanistan and NATO mulls boosting troop levels.

The blast, which NATO said was an improvised explosive device, damaged two of the heavily armored Mine Resistant Ambush Protected vehicles carrying the foreign soldiers and left a small crater in the road, witnesses said.

MRAPs, which are designed to withstand large explosions, are routinely used by international forces moving around Kabul.

At least three civilian cars were also damaged, with one ablaze, while windows even several hundred meters away were shattered. Firefighters and ambulances rushed stunned survivors to hospital.

One security source said that a white Toyota Corolla had exploded as the convoy drove by, though that had not been verified by officials.

Pentagon chief Jim Mattis warned of "another tough year" for both foreign troops and local forces in Afghanistan when he visited Kabul last month.

He would not be drawn on calls by NATO commander in Afghanistan General John Nicholson for a "few thousand" more troops to break the "stalemate" against the insurgents.

But NATO chief Jens Stoltenberg told a German newspaper on Sunday that the 28-nation alliance was considering boosting its troop strength once more given the "challenging" security situation.

The US has around 8,400 troops in the country with about another 5,000 from NATO allies. Most are taking part in NATO's train, assist and advise mission, though some are also carrying out counterterror missions targeting the Islamic State group and al-Qaida.

The Afghan conflict is the longest in US history - US-led NATO troops have been at war there since 2001, after the ousting of the Taliban rule for refusing to hand over Osama bin Laden following the attacks in New York and Washington on Sept, 11, 2001.