New York, New York - Amnesty International urged the US on Monday to investigate possible violations of international law in a deadly strike on a migrant detention facility in rebel-held Yemen.

Last month's attack, which prompted international alarm and was part of the US bombardment campaign against the Yemen's Houthi movement, killed 68 people held at a center for irregular migrants in Saada, the rebel authorities said at the time.

Agnes Callamard, Amnesty's secretary-general, said that "the US attacked a well-known detention facility where the Houthis have been detaining migrants."

The dead were all migrants from African countries.

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Callamard said "the major loss of civilian life in this attack raises serious concerns about whether the US complied with its obligations under international humanitarian law."

"The US must conduct a prompt, independent and transparent investigation into this air strike," she added.

A US defense official had told AFP in the aftermath of the strike that the military launched "battle-damage assessment and inquiry" into "claims of civilian casualties related to the US strikes in Yemen."

Amnesty cited people who work with migrants and refugees in Yemen and visited two hospitals that treated the victims, saying that they had seen "more than two dozen Ethiopian migrants" with severe injuries including amputations.

The morgues at both hospitals had run out of space, the witnesses told Amnesty.

In mid-March, the US began an intense, near-daily bombing campaign against Yemen after the Houthis renewed threats to attack vessels in the vital Red Sea and Gulf of Aden shipping lanes, due to Israel's resumption of its siege of Gaza.

The campaign – which was also at the center of the major "Signalgate" scandal that engulfed President Donald Trump's administration – ended with a US-Houthi ceasefire agreement earlier this month.

Amnesty said it had analyzed satellite imagery and footage from the site of last month's strike on Saada.

The group said it was "unable to conclusively identify a legitimate military target" within the targeted prison compound, citing Houthi restrictions on independent investigations.

"Any attack that fails to distinguish between civilians and civilian objects on the one hand, and legitimate military targets on the other, even within the same compound, constitutes an indiscriminate attack and a violation of international humanitarian law," Amnesty said.