Looking out from a grid of thumbnail-sized pictures, women and children hold placards; forlorn expressions on those with uncovered faces.

"Bring back my son", reads one; "Bring back my brother", says another; and "Bring back my father". All are relatives of men who have been disappeared in Yemen.

The picture has come to me by WhatsApp, sent by a group known as the Abductees' Mothers Association.

They are just a handful of those left in anguish by some of the hundreds of forced disappearances of their loved ones in Yemen, a country in which rebels and pro-government forces have been at war since 2015.

It is a phenomenon that has become increasingly worse in the past four years.

An independent human rights group, Mwatana, has been monitoring cases of arbitrary detention, enforced disappearance, torture and deaths in detention across Yemen, documenting many in a recently released report.

One - 21-year-old Othman Abdo - was snatched nearly four years ago by gunmen in civilian clothing from the courtyard of the mosque next to his house in Hamdan, a district near the rebel-controlled capital, Sanaa.

He was bundled into a car without licence plates and taken to an unknown location.

Othman's family did not know of his whereabouts until they received a phone call from him four months after his disappearance to tell them that he was being held by the rebel-run Political Security Agency in Sanaa.

After many attempts by his relatives, they were allowed to visit him in February 2017.

"When I saw him for the first time, his condition was deplorable, and he was clearly tortured," Othman's mother told Mwatana.

"He had a fractured wrist, with pain in his joints and back."

The young man's relatives said that he had been treated badly and was in urgent need of medical care, which they said was refused by those in charge of the site where he was being held.

His mother said she had to sell the family home to cover the cost of following up the case and to provide for her son while in detention. He was interrogated and accused of collaborating with the Saudi/UAE-led coalition fighting rebels in support of the government, and with the government itself.