"This is the second time that my visit has been postponed, at very short notice," United Nations Special Rapporteur on torture Juan Mendez said in a statement on Wednesday.

"It is effectively a cancellation as no alternative dates were proposed nor is there a future road map to discuss," he said.

Mendez said he had received on April 22 a letter from the Persian Gulf kingdom informing him that the ongoing national dialogue was taking longer than expected and that his visit could impact negatively on the proceedings.

"Due to the sensitivity of my mandate there will never be a perfect time for my visit," the envoy said.

The Bahraini government promptly launched a brutal crackdown on the peaceful protests and called in Saudi-led Arab forces from neighboring states.

Dozens of people have been killed in the crackdown, and the security forces have arrested hundreds, including doctors and nurses accused of treating injured revolutionaries.

Several reports accusing Bahrain of failing to hold senior officials responsible for the 2011 violence were released in recent weeks.

A report published by the Bahrain Independent Commission of Inquiry in November 2011 found that the Al Khalifa regime had used excessive force in the crackdown and accused Manama of torturing political activists, politicians, and protesters.

Bahrainis say they will continue holding demonstrations until their demand for the establishment of a democratically elected government is met.