US policy on drone strikes has been questioned by a rights group who say a strike on a wedding procession killed civilians, not al-Qaeda fighters, as previously claimed by US officials.

Rights group Human Rights Watch (HRW) published a 28-page report on Thursday that said all the victims of a December 2013 drone strike were civilians, citing witnesses and relatives of the victims. The strike in the city of Radaa in Yemen's central province of al-Bayda killed 12 people and seriously injured 14 others.

The findings "raises serious concerns about US forces’ compliance with President Barack Obama’s targeted killing policy," the report said, demanding an investigation.

Though it said the convoy of vehicles that was hit by the strike was a wedding party, the report also said the procession, "may have included members of AQAP (Al-Qaeda in the Arabian Peninsula), although it is not clear who they were or what was their fate.

"However the conflicting accounts, as well as actions of relatives and provincial authorities, suggest that some, if not all those killed and wounded were civilians."

Speaking anonymously to the Associated Press news agency in December, US and Yemeni officials said the strike targeted a mid-level leader of AQAP. Later reports suggested that the intended target, Shawqui Ali Ahmed al-Badani, was wounded and had escaped.

Yemeni officials have said al-Badani was linked to a plot to bomb the US embassy in the capital Sanaa, - a plot that was intercepted by US Intelligence and led to the temporary closure of 19 US embassies across the region in August 2012.

Internal investigation

The US has not officially acknowledged the strike on the wedding party, but NBC News reported that the Obama administration had launched an internal investigation .

Caitlin Hayden, a White House national security spokesperson, told Al Jazeera late on Wednesday that the Yemeni government has said the intended targets of the strike were senior AQAP fighters.

Now, [the US] for many people is a drone, a Hellfire missile and burned bodies and cars. Baraa Shiban, Yemeni human rights researcher.

She reiterated US government policy on drone strikes, announced last May, saying it takes, "extraordinary care to make sure that counterrorism actions are in accordance with all applicable domestic and international law".

"There must be near-certainty that no civilians will be killed or injured – the highest standard we can set. And when we believe that civilians may have been killed, we investigate thoroughly," she said.

Hayden added that condolence payments were made in cases where it was concluded that civilians were killed.

But Letta Tayler, HRW's senior terrorism and counterterrorism researcher and author of the report, told Al Jazeera they had not seen any evidence of US compensation, and that the attack contradicted Obama's policy.

"The likelihood of civilian casualties in this attack raises serious questions about whether US forces are complying with President Obama’s policy that the US only strikes when it has ‘near-certainty’ that no civilians will be harmed," she said.

"We welcome President Obama’s policies but we have no way of knowing if they are being applied in practice," said Tayler.

The rights group published the names of all the victims of that strike and also of those injured, all of them civilians according to relatives.

'Loss of confidence'

Yemeni families affected by the strike have told Al Jazeera that no investigation has been carried out by either government.

Baraa Shiban, a human rights researcher working with UK-based rights group Reprieve, told Al Jazeera the strike "led to the loss of confidence amongst the people with the Yemeni Government."

"Now, [the US] for many people is a drone, a Hellfire missile and burned bodies and cars," he told Al Jazeera.

In a previous interview with Al Jazeera, Baraa, who was the first human rights worker to document the incident, said he received anonymous death threats after his investigation.

White House spokesperson Hayden told Al Jazeera that any investigations into civilian casualties from drone strikes would not be made public but are made available to committees in the US Congress for review.

Footage and photographs taken after the strike show the men were carrying rifles, which is common in some Arab weddings.

Follow Rahul Radhakrishnan on Twitter: @RahulRadhakris