Human Rights Watch (HRW) has called for an end to all weapons sales to Saudi Arabia following the bombing of a school bus last month that killed 51 people, including 40 children.
The rights group called the attack an apparent war crime and said it places arms suppliers at "risk of complicity in war crimes".
HRW's statement, released on Sunday, came just hours after Saudi Arabia and the United Arab Emirates admitted the bombing was "unjustified".
A probe, conducted by the Saudi-led coalition fighting Yemen, concluded on Saturday that "mistakes" were made in the August 9 air raid in Saada province.
The coalition, which initially defended the bombing saying it hit a "legitimate military target", has now pledged to hold to account those responsible for the attack.
HRW said it received photographs and videos of munition fragments that a lawyer based in Sanaa, about 235km south of Saada, said were at the site.
Markings visible on photos and videos of one of the remnants - a guidance fin for a GBU-12 Paveway II bomb, an aerial laser-guided bomb - show it was produced by Pentagon's top weapons supplier Lockheed Martin, according to the rights group.
HRW also said that while it could not confirm the remnants were found at the site of the attack, images of damage from the scene are consistent with the detonation of a large, impact-fused aerial bomb.