The designation, which will be effective in a week, comes after the United States in January declared several other Muslim Brotherhood branches to be terrorist organizations, including in its historic base of Egypt.

"The Sudanese Muslim Brotherhood uses unrestrained violence against civilians to undermine efforts to resolve the conflict in Sudan and advance its violent Islamist ideology," the State Department said in a statement.

The Sudanese Muslim Brotherhood "has contributed upwards of 20,000 fighters to the war in Sudan, many receiving training and other support from Iran's Islamic Revolutionary Guard Corps," the elite ideological wing of Tehran's military, the State Department said.

The State Department accused the Sudanese Muslim Brotherhood of having "conducted mass executions of civilians in areas they captured."

Iran, run by Shiite clerics, and the Muslim Brotherhood, a Sunni organization that historically had extensive social networks inside Egypt, both have supported Sudan's army.

The army has been engaged for nearly three years in a brutal civil war against the paramilitary Rapid Support Force (RSF), with the fighting claiming tens of thousands of lives, displacing more than 11 million people and plunging areas into famine-like conditions.

UN probes have said that the RSF relies on backing from the United Arab Emirates, a close US ally, which has denied providing material support.

The United States last year said that the RSF has carried out acts of genocide with systematic killings and sexual violence against black Sudanese. The United States also said the army carried out war crimes.

Egypt's military ruler turned president, Abdel Fattah al-Sisi, has cracked down hard on the Muslim Brotherhood at home.

Targeting the Muslim Brotherhood has also been a rallying cry in Washington for some conservative Republicans, in part over unfounded conspiracy theories that the group is trying to impose Islamic sharia law in the United States.