UN chief Ban Ki-moon called on Bahrain to respect the human rights of its people and expressed concern about a jailed Shi'ite activist, who led an uprising in the Gulf state last year and has been on a hunger strike for the past 10 weeks.
Activists are pressing for democratic reforms in a country dominated by the ruling Al Khalifa family, a Sunni dynasty that rules a majority Shi'ite population.
Activists protested during the Formula One Grand Prix last week to try and embarrass the rulers while the global focus was on the kingdom for the race.
Abdulhadi al-Khawaja, one of 14 men in prison for leading an uprising last year, has said he will fast to his death if he is not freed and plans to reduce his food intake to just water.
He is serving a life sentence for expressing support last year for Bahrain becoming a republic and has been fasting for 76 days.
UN Secretary-General Ban's office said in a statement that he is "concerned" by al-Khawaja's situation.
"The Secretary-General once again urges the Bahraini authorities to resolve Mr Al-Khawaja's case based on due process and humanitarian considerations without any further delay," the statement said.
Khawaja, who helped found the Bahrain Center for Human Rights, is also well known in the international rights arena.
He has worked with Front Line Defenders and Amnesty International, conducting research in Iraq in 2003.
"The Secretary-General remains concerned about the situation in Bahrain, particularly with regard to the continuing clashes between security forces and protesters which have resulted in more casualties," Ban's office said.
He appealed for all sides to show utmost restraint and to immediately end the violence.
"The Secretary-General also calls on the Bahraini authorities to fully respect the fundamental human rights of the Bahraini people, including due process concerning all detainees," it said.
Bahrain, an American ally that hosts the US Navy's Fifth Fleet, has been in turmoil since the protests began early last year.
Though martial law and Saudi troops were brought in to crush them after one month, the strife has continued with regular mass marches by opposition parties and violent clashes with police.
Foreign governments, rights groups and media watchdogs have
criticized Bahrain for its handling of the protests and the slow
pace of reforms.