Protesters have rallied in Yemen's port city of Aden for the fourth day, angry at the killing of a man who testified against four Emirati-backed fighters accused of raping a seven-year-old boy.

Raafat Danbaa's mother said members of an Emirati-linked militia abducted her son outside their home and later killed him.

"His killers took him right in front of my eyes, outside the front door. They didn't even respect my presence," said the woman, identifying herself as Umm Raafat.

"I was looking at them and I was crying, but they took him anyway… an entire armed unit grabbing one man," she added before begging Yemen's President Abd-Rabbu Mansour Hadi for help in bringing the killers to justice.

"I call on the president to ensure they are brought to justice and that my son finally gets retribution."

Yemen's government has set up a committee to investigate Danbaa's death, which comes against a backdrop of human rights abuses carried out by the UAE-linked militias and the country's armed forces directly.

In June 2018, an Associated Press investigation found that the Emirati military officers ran a prison in southern Yemen, where they tortured detainees, including with acts of sexual abuse.

A report later obtained by Al Jazeera revealed the existence of a network of 27 such sites, used to imprison and torture Yemeni opponents of the UAE.

The report alleged that 49 people had died as a result of the torture they had undergone at the sites.

Yemen has been suffering from a war, which began in 2014 and escalated in 2015 when Houthi rebels swept across much of the country, forcing Hadi to flee and prompting intervention by a Saudi and Emirati-led alliance.

The conflict has destroyed much of the country's vital infrastructure and left it on the brink of famine, with around 11 million people facing starvation.

Death tolls vary significantly, but one observer group put the number of dead at 60,223 since January 2016.