Human Rights Watch on Tuesday urged Yemen to launch an independent probe into the "massacre" of 45 protesters during the 2011 uprising against ousted autocratic president Ali Abdullah Saleh.
The New York-based watchdog said an earlier investigation ordered by Saleh into the killings was flawed, calling for a new probe that is "independent, impartial and meets international standards."
"Several senior former and current government officials appear to have played a role in the massacre but have not been charged," it said in a 69-page report entitled "Unpunished Massacre".
A new investigation must ensure all those implicated in the killings "regardless of position or rank, are arrested and appropriately prosecuted," it stressed.
In the event that Yemen fails to conduct such a probe, HRW urged the United Nations and world powers to "publicly support an independent international investigation into these violations".
HRW said 45 Yemeni protesters, most of them university students and three of them children, were killed in three hours on March 18, 2011, while 200 others were wounded by gunmen loyal to Saleh.
"Security forces made no serious effort to stop the carnage," HRW said.
Seventy-eight suspects were indicted in connection with the killings following the probe ordered by Saleh, but more than half of them remain at large, it added.
The watchdog said the trial of the alleged "killers began in September 2012 but ground to a halt after lawyers for the victims sought top officials' indictments".
"Yemen's transition government is basing its prosecution of the case on a deeply flawed investigation by the Saleh administration," said HRW.
Saleh dismissed the attorney general when he demanded that primary suspects, including government officials, be brought for questioning, it said.
Hundreds of people were killed across Yemen during the year-long uprising of 2011 that pushed Saleh out of power after 33 years of iron-fisted rule.
But Saleh only agreed to step down in November 2011 under a Gulf-brokered and UN-backed deal that gave him and his aides immunity from prosecution.
HRW on Tuesday urged the UN Security Council, the Gulf Cooperation Council, and world powers to "publicly oppose immunity for Yemeni officials implicated in serious violations of international human rights law or international humanitarian law".
It said such immunity must have "no effect in jurisdictions outside of Yemen".
The watchdog also urged "an asset freeze and travel ban on current and former officials implicated in the 'Friday of Dignity' attack and other serious human rights violations until perpetrators are fully and appropriately held to account and victims receive adequate redress".