An air strike on a Yemeni funeral ceremony which killed at least 140 people "is an apparent war crime," Human Rights Watch said on Thursday.
The Saturday strike which Yemen's Houthi rebels blamed on a Saudi-led military coalition was one of the deadliest in the coalition's air campaign against the rebels and their allies.
The coalition has faced mounting international criticism over civilian casualties from its bombing.
"While military personnel and civilian officials involved in the war effort were attending the ceremony, the clear presence of several hundred civilians strongly suggests that the attack was unlawfully disproportionate," Human Rights Watch said in a statement.
"The funeral strike underscores the urgent need for credible international investigations into alleged laws-of-war violations in Yemen."
The watchdog called on foreign governments including the United States and Britain to immediately suspend arms sales to Saudi Arabia.
"After unlawfully attacking schools, markets, hospitals, weddings and homes over the last 19 months, the Saudi-led coalition has now added a funeral to its ever-increasing list of abuses," said HRW's Middle East and North Africa director Sarah Leah Whitson.
"An independent international investigation of this atrocity is needed as the coalition has shown its unwillingness to uphold its legal obligations to credibly investigate."
Leading rebel officers were among those killed in the strike, the rebel-controlled Saba news agency reported.
Saudi Arabia announced an easing of its blockade of rebel-held areas to allow the evacuation of hundreds of wounded for treatment abroad.
The coalition initially denied responsibility but after condemnation from Western governments, it promised an investigation of the "regrettable and painful" event.
A letter sent to the UN Security Council on Sunday "expressed the kingdom's deep regret" over the "attack", state media reported the following day.
"It also renewed its full respect for and commitment to international humanitarian law and international human rights law, and the emphasis on continuing to ensure the taking of all possible measures to protect civilians and civilian sites in Yemen."