At least 11 Afghan civilians including women and children have been killed and 16 injured in a Nato airstrike in the eastern province of Kunar, local security officials have told the BBC.
The airstrike took place in Narang district after local police and soldiers came under attack, they said.
Nato is investigating the air strike, which President Hamid Karzai condemned.
Civilians deaths have caused tension between him and Nato. The deaths come as his successor is being decided.
Both candidates, Abdullah Abdullah and his rival Ashraf Ghani, believe they won June's poll and both alleged widespread fraud.
An audit of votes from the disputed presidential election has been completed, officials say, but results will not be made public for days.
Civilian casualties caused by Nato aircraft are highly sensitive in Afghanistan and have long been a source of tension between President Karzai's government and Nato.
Last year the president banned foreign airstrikes in residential areas after civilians were mistakenly killed in a night raid.
A statement from his office on Wednesday said that US aircraft were responsible for Tuesday's strike.
Kunar provincial governor Shuja ul-Mulk Jalala told the Reuters news agency that at least two airstrikes took place after a group of Afghan police and US forces were attacked in a remote, mountainous area.
Because the area is barely under the government's control, the governor said, it was difficult to ascertain the exact details.
A US military spokesman told Reuters that investigators are "looking into the circumstances" of the second strike.
Correspondents say that the inaccessibility of the area makes it difficult to find out whether those killed were insurgents or civilians.
An Amnesty International report published in August said that while Taliban insurgents were responsible for most civilian deaths, the US had failed properly to investigate fatalities caused by their forces,
Nato said at the time that it takes allegations of civilian casualties extremely seriously and fully investigates all reports.
Mr Karzai has called on Mr Ghani and Mr Abdullah to fulfil a pledge they made last week to create a government of national unity.
Nato hopes the new government will allow some of its forces to stay in Afghanistan after the alliance withdraws combat troops from the country at the end of this year.
Reproduced under licence from BBC News © 2014 BBC