Amnesty International claims it has evidence that points to US involvement in air strikes on suspected al Qaida hideouts in Yemen late last year, and criticised Washington for allegedly using cluster munitions and not taking precautions to avoid civilian casualties.
There are differing accounts of the December 17 attack in the al-Majalah area of the southern Yemeni province of Abyan. Yemeni security officials originally said 34 al Qaida militants were killed, although a Yemeni parliamentary committee later said 41 civilians were killed in the attack as well.
The US has not officially confirmed a role in the air strikes, although US officials have previously acknowledged American involvement in the bombing.
Amnesty said it has five photographs apparently taken after the attack that indicate the use of cruise missiles and cluster munitions.
"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," said Philip Luther, deputy director of Amnesty International's Middle East and North Africa Programme.
Copies of the photographs show twisted scraps of what appear to be missile fragments. The group does not say how it obtained the photographs, and their authenticity could not be immediately verified.
Amnesty said the photos show the payload and body segments of a US-manufactured BGM-109D Tomahawk cruise missile. It said such missiles, which can be launched from a warship or a submarine, are designed to carry a payload of cluster munitions that scatter over a vast area, and are only used by US forces.
"Based on the evidence provided by these photographs, the US government must disclose what role it played in the al-Majalah attack, and all governments involved must show what steps they took to prevent unnecessary deaths and injuries," Mr Luther said.
At least 30 countries have ratified a new international convention banning cluster munitions. The US did not join the treaty.
Yemeni authorities have said their security forces carried out the December strikes, and Washington has not acknowledged a role in them. However, US officials have acknowledged US involvement in the bombing.