UK Human Rights workers Qatar - Two staff members of the Global Network for Rights and Development (GNRD) have been missing since Sunday, the Norway-based non-government organization said.

The UK Human Rights workers Qatar identified as researcher Krishna Upadhyaya, 52, and photographer Gundev Ghimire, 36, had been working on a report on overworked Nepali migrant laborers in the country - which is said to host the 2022 World Cup, when they mysteriously disappeared, BBC has learned.

GNRD said the missing UK Human Rights workers Qatar had been in the country since August 27. It suspects that the men, both of Nepalese origin, had been detained. However, Qatari authorities had reportedly denied having any information.

Prior their disappearance, Upadhyaya, one of the UK Human Rights workers Qatar had reportedly managed to contact a friend in Norway Saturday, to inform that he and Gundev were being harassed and followed by plain-clothes police.

On the day of their supposed departure from Qatar, the duo checked out of the Grand Hyatt but were said to be greatly anxious of the police presence that they decided to stay in the reception area until their evening flight back to Norway.

"He was feeling unsafe to leave the hotel premises and according to him there were many police, most likely undercover ones, coming in very close to him every time he spoke to anyone on the phone. He expressed that he felt very insecure about what might happen when he left the hotel for the airport," read a message to the charity from a friend who was receiving texts from Mr. Upadhyaya.

On Thursday, GNRD released a statement expressing deep concern for the safety of its employees while revealing the legal action it plans to take to secure the men's release.

"It is really regrettable that Qatar, which is signatory state to numerous international law conventions, especially on Enforced Disappearances, is carrying out such explicit violations of human rights," the statement read.

Amnesty International is also urging Qatar to disclose the whereabouts of the UK Human Rights workers Qatar and guarantee their safety.

The gas-rich nation has reportedly long been criticised on its treatment of migrant workers. It has revealed plans to improve workplace safety, housing and pay conditions for expatriate workers earlier this year.

The Qatari embassy in London declined to comment on the disappearance of the UK Human Rights workers Qatar, according to The Independent.