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 it carry out more than 20 lethal strikes on boats suspected to be carrying drugs in the Caribbean and the Pacific.

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

Clan del Golfo is the latest Latin American criminal group to be added to the US Treasury's list of FTOs.

The group has been engaging in criminal activities for decades, mainly trafficking cocaine from Colombia - the largest producer of the drug - to destinations in the US and Europe.

But the Clan del Golfo, which is based in the northern Urabá region of Colombia, also plays a key role smuggling migrants through the Darién Gap, the expanse of jungle linking Colombia to Panama.

In a statement announcing its designation as an FTO, US Secretary of State Marco Rubio said the group was also behind terrorist attacks against public officials, law enforcement and military personnel, and civilians in Colombia.

It is estimated to have thousands of members and is thought to be the largest cocaine-trafficking gang currently operating in Colombia.

It joins three other Colombian criminal groups on the list of FTOs: the left-wing guerrilla group National Liberation Army (ELN), and two groups that broke away from the Revolutionary Armed Forces of Colombia when it signed a peace agreement with the Colombian government in 2016.

The FTO designation of the Clan del Golfo by the US comes less than a fortnight after Colombia's President, Gustavo Petro, signed a landmark agreement with the criminal group aimed at bringing peace to the areas under its control.

Petro campaigned on a promise to bring total peace to the South American country, which has for decades suffered from cartel and guerrilla violence. But that promise has faced considerable challenges and setbacks.

The US designation complicates the peace talks, as it bolsters the US's ability to penalize the group, freezing any assets it may hold in US institutions.

The US has shifted its focus towards Colombia, suggesting potential military actions against drug traffickers on land, emphasizing an ongoing commitment to counter narcotics efforts in the region.