GENEVA (Reuters) - Some 50 prisoners in Iraq face possible execution on Monday following conviction on terrorism-related crimes in unfair trials, United Nations human rights experts said on Friday. The independent experts urged the Baghdad government to immediately halt all mass executions, noting that 21 prisoners had been executed in October, followed by a further 21 last week in Nasiriyah Central Prison, also known as al Hoot. The wave “seems to be part of a larger plan to execute all prisoners on death row”, they said in a joint statement. Some 4,000 prisoners, most of them charged with terrorism offences, are believed to be on death row in Iraq, the experts said. Hundreds of executions are imminent after their execution orders were signed off, they said. “We strongly urge the Iraqi government to respect its international legal obligations and to immediately halt further plans to execute prisoners,” said the U.N. investigators, who focus on torture, arbitrary killings and protecting human rights while countering terrorism. Trials under the Anti-Terrorism Law have been marked by alarming irregularities, they added. “Defendants have frequently been denied the most basic right to an adequate defence and their allegations of torture and ill-treatment during interrogations have not been investigated.”There was no immediate comment from the Iraqi government. U.N. human rights chief Michelle Bachelet, in a statement issued last week after the execution of 21 men, called on Iraqi authorities to halt any further executions. Her office had found “frequent violations of fair trial rights, ineffective legal representation, over-reliance on confessions and frequent allegations of torture”, she said. The administration of justice underpins a functioning, democratic society, Bachelet added.