"Venezuela categorically, firmly, and absolutely rejects the new and ridiculous lie from the Secretary of the Department of State, Marco Rubio, who designates the alleged Cartel of the Suns as a terrorist organization... to justify an illegitimate and illegal intervention against Venezuela," the Ministry of Foreign Affairs said in a statement.

Washington's listing of the "Cartel de los Soles" (Cartel of the Suns) as a foreign terrorist organization officially took effect on Monday, opening the door to new forms of US pressure on leftist President Nicolas Maduro.

US President Donald Trump's administration alleges that the shadowy group is run by Maduro. No evidence has been made public to support the accusation of Maduro's involvement in the group.

The designation is part of a US campaign against drugs and illegal immigration from Latin America.

The US has deployed the world's largest aircraft carrier and other military forces in Caribbean waters as part of the Trump administration's anti-drugs campaign, but officials in Caracas suspect that the United States is mounting an operation to topple Maduro, Venezuela's leftist leader.

US forces have killed at least 83 people in air strikes on boats accused of ferrying drugs in international waters since September, according to an AFP tally of publicly released figures. But no evidence has been made public that drugs were in the boats.

However, with a major military presence now deployed in the Caribbean, including an aircraft carrier, the FTO designation will give legal cover for more pressure on the Venezuelan authorities.

The Cartel of the Suns is responsible for "terrorist violence throughout our hemisphere," Rubio said on November 16.

The Trump administration has been vague about how far it is willing to go in Venezuela, but the huge military build-up and regular killings of people in small boats have rattled nerves -- and prompted concerns in Washington that the US military may be breaking the law.

On Saturday, six airlines announced they were canceling flights to Venezuela due to safety concerns.

The US Federal Aviation Administration on Friday urged civilian aircraft in Venezuelan airspace to "exercise caution" due to the "worsening security situation and heightened military activity in or around Venezuela."