The United Arab Emirates runs at least two secret prisons in Yemen and has reportedly transferred prisoners to a base in Eritrea, Human Rights Watch (HRW) said on Thursday. 

Concerns come as UAE backed militias tasked with fighting the Islamic State group and Al-Qaeda's affiliate inside Yemen were found to be torturing and kidnapping dozens of people during security operations. 

The UAE is a key member of a Saudi-led military coalition that entered Yemen's conflict in 2015 to battle on the government's side against Iran-backed Houthi rebels.

HRW said UAE officials appeared to have "moved high-profile detainees outside the country" including to a base in Eritrea.

The rights group said it had documented 49 cases, including those of four children, who had been "arbitrarily detained or forcibly disappeared" - at least 38 of them by UAE-backed forces.

The New York-based group said the UAE also runs detention facilities in southern provinces home to al-Qaeda in the Arabian Peninsula and a local affiliate of the Islamic State group.

Children are among those detained in the centres, it said. 

It said Shia Houthi rebels and their allies, forces loyal to former president Ali Abdullah Saleh, had also "arbitrarily detained and disappeared scores of people in northern Yemen". 

HRW also accused UAE-backed Yemeni forces tasked with fighting the Islamic State group and al-Qaeda of arbitrarily detaining, kidnapping and torturing dozens of people during their security operations. 

"You don’t effectively fight extremist groups like al-Qaeda or ISIS by disappearing dozens of young men and constantly adding to the number of families with ‘missing’ loved ones in Yemen,” said Sarah Leah Whitson, Middle East director at Human Rights Watch.

“The UAE and its partners should place protecting detainee rights at the centre of their security campaigns if they care about Yemen’s long-term stability.”

Since March 2015, a Saudi-led coalition of states, including the UAE, has conducted an aerial and ground campaign in support of President Abd Rabbuh Mansour Hadi against Houthi forces and its allies, who took over the capital, Sanaa, in September 2014.

The US and UK has provided military support to the coalition.

The World Health Organisation estimates more than 8,000 people have been killed in two years of conflict in Yemen, which also faces a deadly cholera outbreak and the threat of famine. 

All parties in Yemen's war have drawn harsh criticism for causing civilian suffering. 

The United Nations and HRW have said air strikes by the Saudi-led alliance have killed many civilians and may amount to war crimes.

During the conflict, Al-Qaeda in the Arabia Peninsula (AQAP) seized weapons, territory, and revenue by looting the central bank in Mukalla, the capital of the Hadramawt governate, and running the city’s port for about a year, Reuters reported.

AQAP has carried out numerous attacks, primarily on military and security targets in Yemen’s southern and eastern governorates, killing dozens of people.

The local IS affiliate in Yemen (IS-Y) has also claimed responsibility for similar attacks over the last two years.