NATO and US forces in Afghanistan admitted Friday that a number of civilians were killed in two separate airstrikes, days after President Hamid Karzai warned that such deaths threatened relations with the US.

A joint statement by NATO's International Security Assistance Force (ISAF) and US forces gave no details of how many civilians died in each incident, but local officials have put the total at more than 20, including women and children.

"The coalition takes full responsibility for these tragic and regrettable incidents, and we will meet with the family members of those who died or were injured to express our sincere condolences," the statement said.

ISAF commander General John Allen will brief Karzai on the results of initial investigations into the incidents on May 4 in southern Helmand province and on May 6 in northwestern Badghis province.

"The president will be assured of our commitment to take any and all appropriate actions to minimise the likelihood of similar occurrences in the future," the statement said.

"If our investigation finds someone responsible, appropriate action will be taken to hold them accountable."

On Sunday, Karzai summoned Allen and US ambassador Ryan Crocker to warn that civilian deaths caused by international forces fighting a Taliban insurgency threatened the strategic pact he signed with US President Barack Obama last week.

"We must not allow these incidents to damage our shared goals of a peaceful and secure Afghanistan," the ISAF statement said.

"We have accomplished much together over the past ten years. We, as partners with the Afghan government, look forward to continuing this momentum."

In the Helmand incident, six members of a family, including two boys, three girls and a woman, were killed in an airstrike after ISAF checkpoints came under attack, the provincial governor's office said.

In Badghis province, an airstrike killed 15 civilians, including women and children, the provincial member of parliament Qazi Abdul Rahim told AFP, although that figure has not been independently confirmed.

Civilian casualties have always been a sensitive issue in the US-led war against Taliban Islamists, who were ousted from power in a 2001 invasion, and have often been the cause of tense relations between Kabul and Washington.

The number of civilians killed has risen steadily each year for the past five years, reaching a record of 3,021 in 2011, the great majority caused by militants, according to UN statistics.

The president said civilian casualties always hurt Afghan-American relations, adding that Afghanistan had signed the strategic pact with the US to prevent such incidents and safeguard the lives of Afghans.

"If the lives of Afghans are not protected, the strategic partnership will lose its meaning," his office said.

The pact covers relations between the two countries when US-led NATO forces helping Karzai's government fight the insurgency pull out in 2014.

The latest civilian casualties come on top of a series of incidents this year that have rocked relations between the United States and its Afghan allies.

Videos and pictures have emerged of US forces abusing Taliban corpses, copies of the Koran were burnt on a major US military base and an American sergeant has been charged with 17 counts of murder over a massacre of civilians.

NATO forces have also suffered an increasing number of so-called green-on-blue attacks, in which Afghan army troops have turned their weapons against their US-led allies.

In the latest attack, on Friday, one US soldier was killed and two were wounded inside an Afghan-US military base in Kunar province -- taking the death toll this year to 20 in at least 15 separate incidents.

ISAF has around 130,000 soldiers fighting alongside some 350,000 Afghan security personnel who are due to take over responsibility for the fight against the Taliban when US-led forces withdraw.