Sudan’s military has used chemical weapons on at least two occasions against the paramilitary group it is battling for control of the country, four senior U.S. officials said Thursday.

The weapons were deployed recently in remote areas of Sudan and targeted members of the Rapid Support Forces paramilitaries that the army has been fighting since April 2023. But U.S. officials worry the weapons could soon be used in densely populated parts of the capital, Khartoum.

The revelations about chemical weapons use came as the United States announced sanctions Thursday against the Sudanese military chief, Gen. Abdel-Fattah Burhan, for documented atrocities by his troops, including indiscriminate bombing of civilians and the use of starvation as a weapon of war.

The use of chemical weapons crosses yet another boundary in the war between the Sudanese military and the RSF, its former ally. By many measures, the conflict has created the world’s worst humanitarian crisis, with as many as 150,000 people killed, over 11 million displaced and now the world’s worst famine in decades.

“Under Burhan’s leadership, the SAF’s war tactics have included indiscriminate bombing of civilian infrastructure, attacks on schools, markets, and hospitals, and extrajudicial executions,” the Treasury Department said, using an acronym for Sudan’s armed forces.

Burhan responded with defiance: “We are ready to face any sanctions for the sake of serving this nation, and we welcome them,” he told reporters during a visit to El Gezira state.

The U.S. decision is considered a significant move against a figure seen by some as Sudan’s de facto wartime leader, who also represents his country at the United Nations.

Aid groups fear that Sudan’s military could retaliate against the sanctions by further restricting aid operations in areas that are either in famine or sliding toward it. The decision could also reshape broader relations between Sudan and the United States, whose Sudan envoy, Tom Perriello, has been a leading figure in the faltering efforts to reach a peace deal.

Although chemical weapons were not mentioned in the official sanctions notice Thursday, several U.S. officials said they were a key factor in the decision to move against Burhan.

Two officials briefed on the matter said the chemical weapons appeared to use chlorine gas.

When used as a weapon, chlorine can cause lasting damage to human tissue. In confined spaces it can displace breathable air.

Knowledge of the chemical weapons program was limited to a small group inside Sudan’s military, two of the U.S. officials said, speaking on the condition of anonymity to discuss sensitive security matters. But it was clear that Burhan had authorized their use, they said.

Sudan’s ambassador to the United Nations, Al-Harith Idriss al-Harith Mohamed, said in a text message that Sudan’s military had “never used chemical or incendiary weapons.”

‘Militia used them’

“On the contrary, it’s the militia that used them,” he added, referring to the Rapid Support Forces.

Last week, the United States determined that the Rapid Support Forces had committed genocide in the war and imposed sanctions on its leader, Gen. Mohammed Hamdan Dagalo, for his role in atrocities against his own people. The United States also sanctioned seven companies based in the United Arab Emirates that traded in weapons or gold for the RSF.

Sudan’s military has been accused of using chemical weapons before. In 2016, Amnesty International said it had credible evidence of at least 30 likely attacks that killed and maimed hundreds of people, including children, in the western Darfur region. The organization published photos of children covered in lesions and blisters, some vomiting blood or unable to breathe.

As the United States debated punitive measures against Burhan last week, Sudanese authorities announced that they would maintain a major aid corridor through neighboring Chad, a move U.S. officials saw as an effort to avoid the sanctions.

But the evidence of chemical weapons was too compelling to ignore, several U.S. officials said.

The United States detected numerous chemical weapons tests by Sudanese forces this year, as well as two instances in the past four months in which the weapons were used against RSF troops, two officials said.

The United States also obtained intelligence that chemical weapons could soon be used in Bahri, in northern Khartoum, where fierce battles have raged in recent months as the two sides compete for control of the capital.

World War I gas use

Chlorine was first weaponized during World War I, and its use in combat is prohibited by international law. In the mid-2000s, insurgents in Iraq weaponized chlorine in attacks on U.S. troops. It has also been used in improvised bombs by Islamic State fighters and by the Assad regime in Syria.

Officials briefed on the intelligence said the information did not come from the United Arab Emirates, an American ally that is also a staunch supporter of the RSF.

Until Thursday, Sudan’s military was riding high. Last weekend, its troops recaptured the key city of Wad Madani, the capital of Sudan’s breadbasket region, where residents praised the soldiers for ending a yearlong occupation under brutal RSF control.

The victory, combined with the American accusation of genocide against the Rapid Support Forces, suggested that Sudan’s military was finally gaining momentum in a war that it had very recently appeared to be losing.

But in recent days, reports have emerged of vicious reprisals by Sudanese troops against suspected RSF collaborators in the area, including torture and summary executions. The United Nations said it was “shocked” by the reports and ordered an investigation into the killings.

Although the use of chemical weapons was a central element in the decision to level sanctions against Burhan on Thursday, the action was also in response to the military’s bombing raids that have killed dozens of civilians at a time.

Two U.S. officials said the United States was caught in a bind when it came to addressing the chemical weapons with sanctions: In order to protect the source and method of the intelligence used to determine that chemical weapons had been used, the United States did not want to reveal details about the strikes, the officials said.

But U.S. officials also wanted to move against Burhan before President-elect Donald Trump’s inauguration Monday. Under U.S. law, Congress must be notified of the discovery of chemical weapons use, and officials said that members of Congress are expected to be briefed on the issue in a classified hearing next month.

In addition to targeting Burhan, the sanctions announced Thursday also targeted a man described as a Sudanese arms supplier and a company based in Hong Kong. A U.S. official said the company had been used to supply Sudan’s military with Iranian-made drones.