Human Rights Watch says a so-called investigatory body, overseen by Saudi Arabia, has for more than two years been covering up war crimes committed by the kingdom in Yemen.

The rights organization released a 90-page analysis on the investigative work of a body, known as the Joint Incidents Assessment Team (JIAT), which is made up of officials from the countries involved in the military "coalition" in Yemen.

Riyadh launched a war on Yemen in March 2015 in an attempt to reinstate Abd Rabbuh Mansur Hadi, Yemen’s former president and a staunch ally of Riyadh, and crush the popular Houthi Ansarullah movement.

The rights group said on Friday that the investigatory body failed “to provide credible, impartial, and transparent investigations into coalition laws-of-war violations.”

The vast majority of the body’s so-called probes in Yemen, according to the HRW, concluded that any strikes on civilians were “unintentional outcomes of legitimate military operations.”

Middle East director at Human Rights Watch Sarah Leah Whitson said “the coalition has claimed that JIAT was credibly investigating allegedly unlawful airstrikes.”

“But the investigators were doing little more than covering up war crimes,” she added.

At least 31 civilians, mostly children, were killed after a Saudi airstrike hit their vehicle in Yemen’s Hudaydah province on Thursday, only two weeks after a strike by the kingdom hit a school bus in the impoverished country and killed scores of Yemeni children.

Whitson warned countries selling arms to Saudi Arabia, including the United States and the UK, that the "sham investigations do not protect them from being complicit in serious violations in Yemen.”

Human Rights Watch urged Riyadh and its allies to improve their investigatory procedures and “appropriately prosecute people responsible for war crimes” in Yemen.

Many analysts say by backing Saudi Arabia in Yemen, the Trump administration may be exposing itself to being held legally responsible for attacks on civilians.

The Trump administration faces increasing pressure even at home for its support for the kingdom. It has been supplying Saudi Arabia and its allies with advanced arms, intelligence sharing and aerial refueling of warplanes operating in Yemen.

More than 14,700 Yemenis have been killed and thousands more injured since the onset of the Saudi invasion.