American missiles were used in a raid against al-Qaeda militants in Yemen in which women and children died in December, rights group Amnesty International says.
Amnesty has released images taken after the raid that it says show remnants of a US-made Tomahawk cruise missile.
Cluster bombs were also apparently used in the attack, which Amnesty described as "grossly irresponsible".
The US has said its troops gave support for the raid, in Abyan province.
But Yemeni officials have denied any US involvement.
At the end of 2009 Yemen suddenly stepped up its offensive against al-Qaeda militants.
The authorities launched a number of raids, saying intelligence showed that Western targets were in imminent danger.
On 17 December two attacks on militant targets were said to have killed more than 30 militants. The raids were hailed as a big success in Yemen.
US President Barack Obama telephoned his Yemeni counterpart, President Ali Abdullah Saleh, to offer his congratulations.
But Amnesty now says the US in fact supported the raid with cruise missiles.
"A military strike of this kind against alleged militants without an attempt to detain them is at the very least unlawful," said Amnesty's Philip Luther.
"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."
Unnamed US officials have said that elite US troops provided essential support, contradicting Yemeni government claims that it was entirely their operation, says the BBC's Sebastian Usher.
But the US has refused to confirm reports that it had fired cruise missiles - the crux of Amnesty's new allegations.
Analysts say the US is deeply involved in the country's drive against al-Qaeda.
But Yemen's leaders are keen not to appear too closely bound to American interests - one reason why the US has been keeping the extent of its military role in the country under wraps, our correspondent adds.