GAZA CITY (Ma'an) -- Israeli prison authorities issued a decision on Sunday to keep detained engineer Dirar Abu Sisi in solitary confinement, the Palestinian Prisoners Society said.

Abu Sisi, 42, disappeared in February 2011 while traveling on a train in the Ukraine and Israel later announced that it was holding him in Ashkelon jail. His lawyer says Israel has yet to file charges against him.

The engineer, from Gaza, has been held in solitary confinement throughout his detention. Israel pledged to release all prisoners held in isolation as part of a deal to end a mass hunger strike by detainees.

Abu Sisi suffers chronic illnesses and has threatened to refuse medicine if he is kept in isolation.

The Palestinian Prisoners Society called on Israel to end Sisi's solitary confinement and heed their calls to improve prison conditions for sick detainees.