At the time, unidentified US officials told the media that the Pentagon had provided firepower and intelligence to government of Yemen, but US involvement in the strike was never officially confirmed. 

The Amnesty photographs appear to show parts of a BGM-109D Tomahawk cruise missile used to deliver a payload of deadly cluster sub-munitions.

"Amnesty International is gravely concerned by evidence that cluster munitions appear to have been used in Yemen," said Mike Lewis, the group's arms control researcher.

"Cluster munitions have indiscriminate effects and unexploded bomblets threaten lives and livelihoods for years afterwards," he said.

Legal breach

Philip Luther, deputy director of the group's Middle East and North Africa Programme, said that if proven, Washington's involvement would represent a breach of international law.

"A military strike of this kind against alleged militants without an attempt to detain them is at the very least unlawful," said Philip Luther, Deputy Director of Amnesty International's Middle East and North Africa Programme.

"The fact that so many of the victims were actually women and children indicates that the attack was in fact grossly irresponsible, particularly given the likely use of cluster munitions."

The Pentagon declined to comment on the allegations when contacted by Al Jazeera.

Amnesty's allegations are also likely to cause a headache for the government of Yemen, which is reluctant to be seen to be working too closely with the US on counter-terrorism operations within its borders. 

Ginny Hill, director of the Yemen Forum at Chatham House in London, said that Sanaa has tried to distance itself from US foreign policy. 

"Yemeni officials tend to publicly downplay the extent of US collaboration because joint counter-terrorism operations raise uncomfortable questions about the government’s credibility, in a country where public opinion is widely hostile to US foreign policy."

Meanwhile, authorities in Yemen said on Sunday they had arrested more than 30 foreigners during a two month crackdown on alleged al-Qaeda operatives.

Those arrested included an American, a Briton and three people from France, Yemeni security sources told Al Jazeera, adding that the majority of the suspects had been studying Arabic in Sana'a.

The source said that the arrests came after "co-operation" with foreign governments, including the US.