New York, December 23 (RHC)-- Human Rights Watch has called for the U.S. military bombing of a Doctors Without Borders hospital in Kunduz to be investigated as a possible criminal act.

In a letter to U.S. Defense Secretary Ash Carter, Human Rights Watch says: "If the current investigation does not include a criminal inquiry, we call on you to order a criminal investigation."

The attack killed 42 people and wounded dozens more. Doctors Without Borders has called for an independent probe.

Human Rights Watch has also accused the United States of flouting the laws of war by failing to investigate unlawful strikes carried out by the Saudi-led coalition targeting Houthi rebels in Yemen.

The humanitarian organization says the U.S.-backed coalition carried out at least six apparently unlawful airstrikes in residential areas of the Yemeni capital, Sana’a, in September and October, killing 60 civilians.