Dozens of civilians have been killed in Afghanistan after two suicide bombers struck a crowded bazaar and a NATO air strike hit a home, officials have said.

Two NATO soldiers were also killed in a helicopter crash, the cause of which was under investigation, the International Security Assistance Force said.

23 people were killed and 50 others were wounded in the suicide attack in a car park crammed with vehicles supplying the largest NATO base in southern Afghanistan, police said.

A suicide bomber on a motorcycle struck first and as a crowd gathered to help the victims a second bomber walked into their midst and set off explosives strapped to his body, according to Kandahar provincial police chief General Abdul Raziq.

"All casualties are civilians -- not a single military person," he said.

Taliban insurgents claimed responsibility for the bombing, saying on their website that "several dozen of the foreign terrorist forces and their puppets were killed Wednesday in a martyrdom attack".

Hours earlier at least 15 civilians, including women and children, were killed in a NATO air strike on a home in Logar province south of Kabul, police said.

ISAF said "multiple insurgents" were killed in the air strike, which was ordered after troops were attacked "with small-arms fire and a grenade".

However deputy provincial police chief Rais Khan Sadeq Abdulrahimzai said that 18 civilians “including women and children, are dead", adding that seven Taliban insurgents were also killed.

Provincial government spokesman Din Mohammad Darvish said "around 15 civilians are dead" after the attack in the early hours of today.

ISAF said in a statement a "precision" air strike was called in after coalition forces were fired on during an operation to detain a leader of the hardline Islamist Taliban insurgents.

"As a result of the operation, multiple insurgents were killed and the Afghan and coalition security force seized several weapons and a quantity of explosives," ISAF said.

An ISAF spokesman later said, after allegations of civilian deaths surfaced, that they were "assessing and gathering facts to try to determine what happened".

Civilian casualties caused by NATO have roiled relations between Afghan President Hamid Karzai and the United States, which leads NATO forces in the fight against the Taliban.

A little over a week ago, Karzai ordered an investigation after Afghan officials said a NATO air strike killed a family of eight, including six children, in eastern Afghanistan.

Two weeks before that, Karzai summoned ISAF commander General John Allen and US ambassador Ryan Crocker to the presidential palace after a number of civilians were killed in other NATO air strikes.

For the past five years the number of civilians killed in the war has risen steadily, reaching a record of 3,021 in 2011, with the vast majority caused by insurgents, the United Nations says.

Kandahar Air Base is the largest NATO military base in southern Afghanistan, which has been a flashpoint for the insurgency over the past decade.

The Taliban have in the past threatened to kill truckers working for NATO, which relies on civilian vehicles to supply their bases across Afghanistan.

The lorries go through complex security checks that can take days, so that dozens of trucks often mass outside military bases before being allowed inside to offload their cargo, attracting makeshift bazaars as they wait.