GENEVA — The United Nations said Friday that it was appalled by a mass execution of prisoners in Iraq and called for an immediate halt to executions, citing flaws in the country’s criminal justice system.

Iraq’s Ministry of Justice said 38 prisoners were executed on terrorism-related charges in the southern city of Nasiriya on Thursday, but it gave no details of the prisoners’ identities or the offenses for which they were sentenced to death.

“Given the flaws of the Iraqi justice system, it appears extremely doubtful that strict due process and fair trial guarantees were followed in these 38 cases,” Elizabeth Throssell, a spokeswoman for the United Nations human rights office in Geneva, told reporters. “This raises the prospect of irreversible miscarriages of justice and violations of the right to life.”

Iraqi courts are dealing with thousands of prisoners, including foreigners and hundreds of children, some as young as 13, who were detained in the past two years because they were suspected of being Islamic State fighters or supporters.

Gruesome accounts of summary executions and the brutal torture of detainees suspected of being Islamic State fighters underscore the powerful impulse for revenge against the militants, who carried out atrocities in the areas they seized after 2014.

United Nations human rights officials have said that speeding up the execution of accused militants could result in the deaths of innocent people. They warn that perceptions of injustice risk deepening the antagonism between Sunni and Shiite Muslims, and lay the foundation for another cycle of sectarian violence.

Thursday’s executions raised the number of people believed to have been executed this year to 106, Ms. Throssell said, but the actual number could be much higher.

United Nations officials learned of the executions from a statement posted on the Ministry of Justice’s Facebook page, she said. The government has stopped providing information on executions, and human rights investigators suspect that many go unrecorded.

Iraqi authorities disclosed 88 executions in 2016, but the number could have been as high as 116, Ms. Throssell said. Human rights groups fear that the pace of executions is set to rise.

Thursday’s executions were the largest number in Iraq on a single day since September, when 42 people were hanged in the same prison in Nasiriya.

The prison is believed to have about 6,000 prisoners on death row, Agnes Callamard, a United Nations human rights expert monitoring extrajudicial executions, said in an email.

Iraq’s judicial authorities have tried or convicted at least 7,374 people on suspicion of Islamic State links since 2014, Human Rights Watch said this month in a report.

A judge in one Iraqi province told the report’s researchers that a counterterrorism court, established to try prisoners seized in the battle to recapture Mosul, had started trials of more than 5,500 people and had sentenced 200 in a six-month period that ended in August.

Iraqi authorities have a right to prosecute militants’ crimes to protect public security, but the judicial procedures are flawed, Human Rights Watch said.

“Everybody has a public defense lawyer, but it does not appear that they are engaging in the trial,” said Belkis Wille, Human Rights Watch’s senior Iraq researcher. “They are sitting there because it’s required by Iraqi law; they are not sitting there because they are providing a defense.”

Iraqi authorities appeared to be prosecuting suspects under counterterrorism laws that impose harsh sentences, including the death penalty and life imprisonment, for membership in the Islamic State, without taking into account the gravity of the offenses they are accused of committing, Human Rights Watch said.

There was no difference between a cook for the Islamic State or a fighter, a counterterrorism court judge told Ms. Wille. “A fighter couldn’t go out and kill the next morning if he hadn’t had a good meal the night before,” she said the judge told her. “So they’re both equally culpable.”

The Islamic State would never have come into existence if the United States military had executed detainees in its Camp Bucca prison, Ms. Wille said the counterterrorism judge told her. He went on to remark, she said, “This time around we need to make sure we kill them all.”