The United Nations secretary general warned Tuesday that the humanitarian situation in Sudan is “growing worse by the day” after a paramilitary force seized the Darfur city of el-Fasher.

Speaking at a U.N. summit in Qatar, António Guterres offered a stark warning about el-Fasher and called for an immediate ceasefire in the two-year conflict that's become one of the world's worst humanitarian crises.

“Hundreds of thousands of civilians are trapped by this siege,” Guterres said.

“People are dying of malnutrition, disease and violence. And we are hearing continued reports of violations of international humanitarian law and human rights.”

He added that there also were “credible reports of widespread executions since the Rapid Support Forces entered the city.”

U.N. officials have warned of a rampage by the paramilitary Rapid Support Forces after it took over the city of el-Fasher, reportedly killing more than 450 people in a hospital and carrying out ethnically targeted killings of civilians and sexual assaults.

The RSF has denied committing atrocities, but testimonies from those fleeing, online videos and satellite images offer an apocalyptic vision of the aftermath of their attack.

The full scope of the violence remains unclear because communications are poor in the region.

The RSF besieged el-Fasher for 18 months, cutting off much of the food and other supplies to tens of thousands of people. Last week, the paramilitary group seized the city.

Asked if he thought there was a role for international peacekeepers in Sudan, Guterres said it was important to gather and pressure countries with influence in Sudan.