The United States has sanctioned four people and four companies accused of enlisting Colombian mercenaries to fight for and train a Sudanese paramilitary group accused by Washington of committing genocide.

Announcing the sanctions on Tuesday, the US treasury said the network was largely composed of Colombian nationals and companies.

Hundreds of former Colombian military personnel have travelled to Sudan to fight alongside the Rapid Support Forces (RSF), which has committed horrific war crimes including ethnically targeted slaughter and large-scale abductions.

The Colombians’ involvement first emerged last year, when an investigation by the Bogotá-based outlet La Silla Vacía found that more than 300 former soldiers had been contracted to fight – prompting an unprecedented apology by Colombia’s foreign ministry.

Colombian ex-soldiers have long been considered among the world’s most sought-after mercenaries due to their extensive battlefield experience gleaned from the country’s decades of civil war, knowledge of Nato equipment, and high-level combat training.

In Sudan, the Colombians have reportedly trained child soldiers, taught fighters to pilot drones, and fought directly on the frontlines. One of the mercenaries told the Guardian and La Silla Vacía in October that he had trained children in Sudan and fought in the siege of the city of El Fasher. He said training the children was “awful and crazy” but added that “unfortunately that’s how war is”.

Among those targeted was Álvaro Andrés Quijano Becerra, a dual Colombian-Italian national and retired Colombian military officer based in the United Arab Emirates. The treasury accused him of playing a central role in recruiting and deploying former Colombian soldiers to Sudan. His wife, Claudia Viviana Oliveros Forero, was also sanctioned.

Also on the sanctions list was Mateo Andrés Duque Botero, a dual Colombian-Spanish citizen who the treasury said managed a business accused of handling funds and payroll for the network that hired the Colombian fighters. “In 2024 and 2025, US-based firms associated with Duque engaged in numerous wire transfers, totalling millions of US dollars,” the treasury statement said.

Colombian national Mónica Muñoz Ucros was the fourth individual to be sanctioned, with the company she managed accused of carrying out wire transfers linked to Duque and his businesses.

“The United States again calls on external actors to cease providing financial and military support to the belligerents,” the treasury said in a statement.

Elizabeth Dickinson, senior analyst at the International Crisis Group, described the sanctions as a “very significant” milestone, saying that “calling out those who are doing the contracting is the right way to go”.

She added that Colombia also recently passed a law ratifying the International Convention Against the Recruitment and Use of Mercenaries, aiming to curb decades of Colombian involvement in foreign conflicts and transform national security policy.

Sean McFate, an expert on mercenaries, urged more caution, saying that “sanctions are necessary but insufficient for dealing with rampant mercenarism”.

“It’s an illicit economy and based out of Dubai, which is relatively sanction-proof,” he said. The UAE has been widely accused of arming the RSF, an accusation it has denied. “Expect more Colombian mercenaries,” McFate warned.