The United Nations is once again being questioned for virtually turning a blind eye on the "historical atrocities" being carried out by the Islamic State (ISIS) against Christians in the Middle East.

In a letter recently sent to Nikki Haley, the permanent representative of the United States to the U.N., the American Center for Law and Justice (ACLJ) slammed the U.N. for refusing to address and take action against the ISIS for its beheadings, sex slavery, and crucifixion of Christians, which it said constituted war crimes.

"We must act to stop ... the evil actions taken by ISIS to intimidate and eliminate Christians and other religious minorities from the face of the Earth," the conservative law group said in the letter.

"The United Nations must lead the world in meeting our international legal and moral obligations," it added.

The ACLJ also called on the U.S. government to press its allies to take action to stop the ISIS campaign of genocide.

The U.N. has reportedly acknowledged that ISIS is committing genocide against Yazidis, but it has not included Christians as among the genocide victims.

Last month, the Alliance Defending Freedom (ADF) International also blasted the U.N. for its failure to uphold human rights, saying it has failed to fulfil its obligations to uphold religious freedom, thus exacerbating the suffering of persecuted Christians worldwide.

In a report bluntly titled "The UN's Failure to Promote and Protect Religious Freedom," the ADF found it particularly "glaring" that the U.N. Security Council held a special meeting in 2015 on ISIS' crimes against approximately 30 lesbian, gay, bisexual and transgender individuals. And yet the world body did not even bother to discuss the more than 10,000 Christians and others who had been slaughtered by ISIS.

The ADF report also questioned the membership of the U.N.'s Human Rights Council, which includes Saudi Arabia and China – "states that perpetuate human rights abuses, including abuses of the right to freedom of religion," according to World Watch Monitor.

In fact, of the U.N. rights body's 47 members, 13 are included in this year's World Watch List of countries where Christians face the most severe persecution, as compiled by the Christian persecution watchdog Open Doors USA.