The United States has designated a notorious drug-trafficking organisation in Colombia as a terrorist group.

The US Treasury Department added the group, known as Clan del Golfo or Gulf Clan, to its list of Foreign Terrorist Organisations (FTOs).

The designation came just hours after US President Donald Trump signed an executive order classifying the drug fentanyl as a weapon of mass destruction.

The two moves are seen as a further ramping-up of the Trump administration's war on drugs, which has also seen over 20 lethal strikes on boats suspected of drug trafficking in the Caribbean and the Pacific.

More than 90 people were killed in the strikes, which legal experts say may breach the law.

Clan del Golfo has been involved in cocaine trafficking from Colombia, the world's largest producer of the drug, to destinations in the United States and Europe. Additionally, it plays a crucial role in smuggling migrants through the Darién Gap, connecting Colombia to Panama.

Following its FTO designation, U.S. Secretary of State Marco Rubio stated that the group is also responsible for terrorist attacks against public officials, law enforcement, military personnel, and civilians in Colombia.

The Clan has thousands of members and is currently the largest cocaine-trafficking gang in the country.

Notably, this designation comes a fortnight after Colombian President Gustavo Petro signed an agreement with the Clan del Golfo aimed at bringing peace in regions under its control. The plan included terms that members would not be extradited to the US.

This new designation by the US complicates these negotiations, and it marks an ongoing tension between the Colombian government and the Trump administration.

Petro's previous promises to usher in "total peace" in Colombia have struggled in the face of significant violence, including cartel and guerrilla conflicts.

The FTO designation gives the US government broader powers to address the situation, including freezing any assets the Clan may hold in US financial institutions. The designation also allows for the prosecution of anyone providing material support to the group.

Trump's administration has linked the deadly opioid fentanyl to the crisis facing the US, emphasizing that ongoing strikes against drug smugglers can save American lives. However, experts question the focus on Colombia as neither Colombia nor Venezuela are major producers of fentanyl. Critics argue the necessity of addressing the root causes of drug use.