A top human rights lawyer in Equatorial Guinea has gone missing, fuelling concern that he has been illegally detained, a rights group has said. Fabian Nsue Nguema was last known to have visited a jail on Monday to see a client, Human Rights Watch (HRW) said. Equatorial Guinea's government has not yet commented on his alleged detention. Mr Nsue represented foreign mercenaries, including UK citizen Simon Man - who was jailed in 2008 for plotting to overthrow the president. Rights groups say Equatorial Guinea is one of the most repressive regime's in Africa. In March, the UN agency, Unesco, was forced to drop the name of Equatorial Guinea's President Teodor Obiang Nguema from a science prize that he had funded, following an outcry by human rights campaigners, African intellectuals and Western governments. Mr Obiang has ruled Equatorial Guinea since seizing power in a coup in 1979. His government has been repeatedly accused of illegally detaining and torturing opponents - allegations it denies. 'Secret detention' "Fabian Nsue's disappearance while visiting a prison is of grave concern," said Daniel Bekele, Africa director at HRW. "The government needs to urgently investigate the situation, determine whether he is being held in secret detention contrary to national and international law and publicly clarify his whereabouts." Mr Nsue - who is also an opposition activist - went to the notorious Black Beach prison in Malabo to meet detained client Augustin Nzogo on Monday, HRW said. "In the late afternoon, Mr Nsue spoke by phone with an acquaintance to say that he was still waiting to see his client. At approximately 5pm [local time, 16:00GMT], those trying to reach him discovered that his phone had been cut off. Mr Nsue failed to return home that evening," HRW said, in a statement. "Sources close to Nsue informed HRW that those who went to the prison and asked for him were refused access. However, one family member was able to gain access to the prison and said they had seen him there, in detention," HRW said, adding that it had not been able to speak directly to the family member. Mr Nsue has been harassed by the government on numerous occasions and was tortured while in custody in 2002, HRW said.