"The United States calls on the Government of Sudan to cease all chemical weapons use and uphold its obligations" under the Chemical Weapons Convention, an international treaty signed by nearly all countries that prohibits their use, the State Department said.

"The United States remains fully committed to hold to account those responsible for contributing to chemical weapons proliferation," State Department spokeswoman Tammy Bruce said in a statement.

The State Department did not immediately offer details on when and where the weapons were used.

The New York Times reported in January that Sudan's military had used chemical weapons on at least two occasions in remote areas its war with the paramilitary Rapid Support Forces (RSF).

Citing anonymous US officials, the newspaper said that the weapon appeared to be chlorine gas, which can cause severe respiratory pain and death.

The State Department said it notified Congress on Thursday of its determination on the use of chemical weapons, triggering sanctions after 15 days.

The sanctions include restrictions on US exports and financing to Sudan's government.

In practical terms, the effect will be limited as both Sudan's military chief Abdel Fattah al-Burhan and his adversary, RSF leader Mohamed Hamdan Daglo, are already under US sanctions.

A power struggle between the army and RSF erupted into full-scale war in April 2023 with devastating consequences for the already impoverished country.

The conflict has killed tens of thousands of people, displaced 13 million and created what the United Nations describes as the world's worst humanitarian crisis.

The army said on Tuesday it had dislodged the RSF from their last positions in Omdurman, part of the Sudanese capital, securing all of Khartoum state nearly two months after recapturing the capital's center.