DUBAI, July 30 (Reuters) - A prominent imprisoned Bahraini opposition figure has lost 10 kg during the first three weeks of a hunger strike, a family member said on Friday, as rights groups called for his release and the return of a confiscated manuscript he had written. Abduljalil al-Singace was sentenced to life imprisonment in 2011 alongside a number of other Shi"ite Muslim activists and opposition leaders for their roles in an uprising in the Gulf island state ruled by a U.S.-backed, Sunni Muslim monarchy. Singace, who also staged a 10-month hunger strike in 2015-2016, has refused food since July 8, demanding better treatment and the release of 12 confiscated notebooks, which his family says hold the manuscript of a book on Arabic dialects. "He resorted to a hunger strike as the last means to end the degrading treatment," the family member said, adding the 59-year old"s voice had weakened and relatives were concerned about his mental health. Bahrain"s government, in response to a request for comment, said Singace"s documents had been "confiscated during an attempt to smuggle them from prison in breach of the established procedures and rules". Singace was being seen daily by doctors and given nutrients with consent, it said, adding that the authorities had advised him to end the hunger strike and await a legal process regarding the manuscript. It said detainees receive healthcare equivalent to that provided by the public health system outside prison, and there was a "zero-tolerance policy" towards mistreatment. The United Nations special rapporteur, Amnesty International and Human Rights Watch, among others, have called for Singace"s release. Western-allied Bahrain has come under pressure from human rights organisations over prison conditions including overcrowding, poor sanitation and lack of medical care. Bahrain"s government says it has made a number of key reforms including an ombudsman who independently investigates allegations of mistreatment. According to the family member, Singace passed his notebooks in April to a prisoner being released, in the hope that the manuscript could be turned over to his relatives. Singace would not stop his hunger strike until his demands were met, the family member said, noting that he had maintained his previous hunger strike for 313 days. The government said Singace was conscious and receiving daily health checks, fluids and nutrition with his consent. "His internal organs are healthy and stable," the statement said. Around 100 academics across the world signed a petition this week calling for Singace"s release and the return of his manuscript. Separately, 16 human rights groups including Amnesty International, Scholars at Risk, the Gulf Centre for Human Rights and the Bahrain Institute for Rights and Democracy (BIRD) also issued a statement with similar demands. "(Singace) has now spent a decade in prison for his peaceful resistance to Bahrain"s dictatorship and it is time for Bahraini authorities to end this petty persecution, return (his) book and order his immediate and unconditional release," said BIRD"s Sayed Ahmed Alwadaei, a Bahraini activist living in exile.