GENEVA (Reuters) - The top U.N. human rights official said on Wednesday he was  appalled  that Iraq had hanged 42 men on Sunday, almost certainly without a fair trial, and that he feared more would follow. The executed prisoners had been convicted of terrorism charges ranging from killing members of the security forces to detonating car bombs.  I am appalled to learn of the execution of 42 prisoners in a single day,  U.N. High Commissioner for Human Rights Zeid Ra ad Al Hussein said in a statement.  We are extremely concerned at reports that Iraq may be planning to expedite the process of executing prisoners already sentenced to death, and that this could result in more large-scale executions in the coming weeks.  Zeid said it was  extremely doubtful  that strict due process and fair trial guarantees, including the men s rights to effective legal assistance and a full appeals process as well as to seek pardon or commutation of their sentence, had been met in every one of the 42 individual cases. The hangings came after Sunni suicide attacks killed at least 60 people near the southern city of Nassiriya, a Shi ite area, on Sept. 14, prompting Shi ite demands for tougher judicial action. Iraqi officials have said that about 1,200 of the estimated 6,000 prisoners held in Nassiriya have been sentenced to death, the statement said. Zeid said Iraq s use of the death penalty raised  massive concerns  and he called on the government to establish an immediate moratorium on its use. Members of terrorist groups who were proven to have committed serious crimes should be held fully accountable, he said.  However, Iraq s use of anti-terrorism legislation to impose the death penalty for a wide range of acts does not appear to meet the strict threshold of  most serious crimes .  No information about those hanged on Sunday has been released, such as their names, places of residence, crimes, trials, or date of sentencing, the statement said. Iraqi officials have said all their appeals processes had been exhausted, the statement said. 