Two British human rights workers investigating the treatment of migrant workers have gone missing in Qatar, after one of them reported being harassed by police, according to the Norwegian human rights group that employs them.

The Global Network for Rights and Development (GNRD), said its staff, British human rights researcher, Krishna Upadhyaya and photographer Ghimire Gundev, were missing since Sunday while researching migrant worker issues in the Gulf nation.

The project manager for the Norway-based rights group accused Qatari security services of being behind the disappearance.

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 Global Network for Rights and Development,

Qatari officials could not immediately be reached for comment.

In a statement released on Thursday, GNRD said it was deeply concerned about the safety of its employees, and it was preparing to take legal action 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," it said.

Meanwhile, Amnesty International urged Qatar to reveal the whereabouts of the British pair and guarantee their safety.

An official at the British Embassy in Doha said the mission was aware of reports that the two British nationals had been detained by Qatari authorities and was investigating the matter.

According to the rights group, Upadhyaya contacted a friend in Norway on Saturday to say he and Gundev were being harassed and followed by plain-clothes police.

He was working on a report on overworked Nepali migrant workers, according to a statement issued by his family.

Labour rights activists have highlighted the unsafe working conditions in the construction industry in the gas-rich nation.

Earlier this year, Qatar unveiled plans to improve workplace safety, housing and pay conditions for expatriate workers.