The US Treasury has frozen the assets of four Colombian paramilitary members, accusing them of drug trafficking.
The paramilitaries are believed to have killed thousands of people
The decision also bans US citizens from doing business with the men.
The move could complicate a peace deal between paramilitaries and Colombia's government, under which fighters are eligible to serve reduced jail terms.
The four are leading members of what is thought to be Colombia's largest paramilitary group, the United Self-Defence Forces of Colombia (AUC).
The Treasury's Office of Foreign Assets Control (Ofac) added twin brothers Miguel Angel and Victor Manuel Mejia Munera, as well as Ramiro Vanoy Murillo and Francisco Javier Zuluaga Lindo to its list of "specially designated narcotics traffickers".
Ramiro Vanoy Murillo and Francisco Javier Zuluaga - who were indicted on narcotics trafficking charges by the US in 1999 - are in a Colombian jail waiting to be extradited to the US.
They are participating in the peace deal, under which they could receive reduced jail terms in Colombia.
The US indicted the fugitive Mejia Munera brothers on narcotics trafficking charges in 2004.
They have abandoned the peace process and are on the run. The US is offering a $5m (£2.5m) reward for information leading to the arrest of Miguel Angel Mejia Munera.
"The Mejia Munera brothers are violent fugitives from justice," said Ofac Director Adam J Szubin.
"Today's action furthers Ofac's effort to destabilise and undermine the operations of these Colombian traffickers."
The listing also bans US citizens from doing business with more than 1,530 businesses and people connected to the four men.
Under the 2003 peace deal, paramilitary leaders surrendered and demobilised 31,000 members to the Colombian government in exchange for reduced jail terms and extradition protection.
The paramilitaries were created to combat rebel armies but evolved into drug-trafficking cartels accused of committing some of the country's worst atrocities.
Correspondents say they are thought to have murdered many thousands of people in their bloody rampage of the past 20 years.