A US air strike on a Syrian mosque that left nearly 40 people dead was “likely unlawful”, a damning new report by Human Rights Watch has found.

“Adequate precautions” to avoid civilian casualties were not taken by American authorities before they dropped two bombs in western Aleppo on 16 March, it says.

The 16-page report found officials were unaware the building was a mosque where evening prayer was about to begin, although proper analysis could have established this.

“The US seems to have gotten several things fundamentally wrong in this attack, and dozens of civilians paid the price,” said Human Rights Watch’s deputy emergencies director Ole Solvang.

“The US authorities need to figure out what went wrong, start doing their homework before they launch attacks, and make sure it doesn’t happen again.”

US officials said the strike was targeting an al Qaeda meeting in a partially constructed community hall. However, Human Rights Watch found no evidence to support the allegation that armed group members were meeting.

Authorities have since said they will investigate whether civilians were caught up in the attack and whether the target hit was part of a complex owned by a mosque.

The report comes shortly after Donald Trump launched a barrage of missiles on a Syrian airbase following a suspected chemical weapons attack on civilians - representing a break from the previous administration’s foreign policy.

The US President ordered the firing of 59 Tomahawk Cruise missiles from the USS Porter and USS Rossinto the Sharyat airfield in western Homs in retaliation to the use of chemical weapons on civilians, allegedly by Bashar al-Assad’s dictatorial regime.