Yemeni air raids and strikes by U.S. drones have been a frequent tactic in Yemen's struggle against Al-Qaeda fighters holed up in remote areas. Yahya Arhab/EPA

The strike on the wedding convoy was the highest-profile attack in Yemen since Obama's major national security speech in May last year, in which he claimed that before authorizing an airstrike outside an active war zone "there must be near-certainty that no civilians will be killed or injured — the highest standard we can set."

The HRW report will likely give ammunition to critics, who say the administration of President Barack Obama operates the drone programs under extreme secrecy and unclear legal status, especially as news reports recently surfaced that the White House is considering using a drone to kill a U.S. citizen living in Pakistan who is allegedly affiliated with Al-Qaeda.

It also follows an investigation by Al Jazeera which interviewed Yemeni witnesses to the event and survivors from the convoy, who testified that the incident targeted a civilian wedding convoy near the town of Radda' in the central province of al-Baydah.

The report comes just months after two similar investigations into the drone programs in Yemen and Pakistan — by HRW and Amnesty International, respectively — both of which allege some U.S. strikes have violated the laws of war .

The Human Rights Watch (HRW) report said that some or all of the men killed in the Dec. 12, 2013, incident might have been civilians, contrary to initial claims from Yemeni officials that the men were fighters. The U.S. has not officially acknowledged carrying out the attack.

A missile strike allegedly carried out by the United States in Yemen last year — which hit a wedding convoy, killing twelve men and wounding 15 others — may have been a violation of the laws of war, a leading human rights organization said in a new report.

'Militants' or civilians?

It appears that official guidelines may not have been followed in the case of the Yemen strike, according to the author of the new HRW report.

“This strike raises serious concerns that U.S. forces are not following President Obama's policy on targeted killings,” says Letta Tayler, a senior researcher at HRW. “We cannot be 100 percent certain that those killed were civilians, but we find it highly probable that all of those killed and injured were civilians, or that some of those were civilians.”

White House spokesperson Caitlin Hayden said she'd not yet read the report, but noted, "the Yemeni Government has stated that the targets of the operation on December 12 were dangerous senior Al-Qaeda militants." Hayden added: "When we believe that civilians may have been killed, we investigate thoroughly."

The State Department did not respond to a request for comment.

The apparent target of the attack was Shawqi Ali Ahmad al-Badani, accused of being the mastermind of a plot that resulted in the temporary closure of 19 US embassies. The Al Jazeera report also suggested another target was Nasr al-Hattam, a local figure with alleged ties to armed groups, but said it was “unclear what imminent threat he posed to the U.S.”

HRW has called for a full investigation into the strike, and recommended that the administration publish the results to the greatest extent possible. They also call for the U.S. to “explain the full legal basis” and “publicly clarify all policy guidelines” for all targeted killings, which remain largely classified.

Since 9/11, the United States has carried out semi-covert airstrike operations in Yemen, Pakistan and Somalia, often through the use of drones. In Yemen, parallel programs are run by the CIA and a secretive arm of the U.S. military called Joint Special Operations Command (JSOC). The CIA also runs a drone program on the border of Pakistan and Afghanistan, a program whose future could be disrupted by the likely drawdown of most U.S. forces from Afghanistan by the end of this year.

U.S. officials began acknowledging the existence of the drone programs in 2010, but little is known about the legal justifications, internal count of civilian casualties, or accountability measures in place. “The first thing the Obama administration needs to do is publicly acknowledge this strike,” said Tayler. “It also needs to confirm that it is investigating this strike, and it needs to publish its findings.” NBC News previously reported that the White House had launched an internal investigation into the strike, quoting anonymous officials.

If all 12 of the dead in the wedding convoy were indeed civilians, it would make the strike one of the deadliest for civilians in Yemen during Obama's presidency. A September 2012 strike, in roughly the same part of the country, killed 12 civilians, including three children and a pregnant woman. A 2009 attack in al-Majalah killed 41 civilians, 21 of whom were children, in a Bedouin camp, and 14 alleged Al-Qaeda fighters.

Although they remain as controversial as ever, the frequency of drone strikes in Pakistan and Yemen declined markedly in 2013 compared to previous years. The U.K.-based Bureau of Investigative Journalism, which studies drone attacks closely, recently noted that the "CIA hadn't bombed Pakistan for 55 days, the longest pause between drone strikes of Obama's presidency."