At least three gangs have struck deals to fly drugs to West Africa and from there to Europe, according to US indictments. One trafficker claimed he already had six aircraft flying. Another said he was managing five airplanes.

Because there is no radar coverage over the ocean, big planes can cross the Atlantic virtually undetected.

"The sky's the limit," one Sierra Leone trafficker boasted to a Drug Enforcement Administration informant, according to court documents.

The new air route is remarkable because of the distances involved and the complexity of flying big jets, said Scott Decker, a criminology professor at Arizona State University who studies smuggling methods.

A trip from Venezuela to West Africa is about 5 470km - about triple the distance from Venezuela to Florida.

The UN Office on Drugs and Crime began warning about trans-Atlantic drug planes after November 2 2009, when a burned-out Boeing 727 was found in the desert in Mali. Drug smugglers had flown the jet from Venezuela, unloaded it and then torched the aircraft, investigators said.

In some cases, executive jets have been used, including a Gulfstream II that landed in Guinea-Bissau in 2008 and another Gulfstream seized in 2007 as it tried to depart Venezuela for Sierra Leone.

In the last year, a flurry of arrests has begun shedding light on how the air routes work. The cases are being prosecuted in a New York federal court because some of the cocaine was supposed to have been sent to the US.

"The quantity of cocaine distributed and the means employed to distribute it were extraordinary," prosecutors wrote in one case.

They warned of a conspiracy to "spread vast quantities of cocaine throughout the world by way of cargo airplanes".

In some ways it is a throwback to the 1970s and '80s, when drug pilots flew freely between Colombia and staging areas near the US border, Decker said. Back then, drug lords such as Amado Carrillo, nicknamed The Lord of the Skies, sent jets with as much as 15 tons of cocaine from Colombia to northern Mexico.

Recent US court cases involving trans-Atlantic flights include:

- The Valencia-Arbelaez Organisation, broken up by undercover US agents after it bought a $2m plane to run monthly flights between Venezuela and Guinea. The group claimed to have six aircraft already flying between South America and West Africa.

- A ring based in Colombia and Liberia, arrested after one of its planes was seized in May with two tons of cocaine as it prepared to leave Venezuela. Prosecutors say the group was planning to fly jets twice a month. One defendant claimed to manage five other aircraft making similar hauls.

- Three Sierra Leone men, accused of scouting out airstrips and arranging for a four-ton flight of cocaine from South America in March.

Two other recent cases have involved cocaine and cargo jets, though investigators have not revealed yet whether the flights were going to Africa:

- Francisco Gonzalez Uribe, a Colombian trafficker due to be sentenced this month. Gonzalez Uribe was recorded while trying to purchase large aircraft including a DC-8, a four-engine jet.

- Walid Makled-Garcia, who prosecutors say controlled airstrips in Venezuela used to launch drug flights. Prosecutors say Makled-Garcia was behind one of the biggest drug plane shipments in recent years: a DC-9 that landed in Mexico in 2006 with more than 5.6kg of cocaine on board.

All five cases are being prosecuted in a federal court in Manhattan.

Several factors have made trans-Atlantic air routes more attractive, said Carlos Moreno, an expert on trafficking at Icesi University in Cali, Colombia.

Cocaine use has been rising over the last decade in Europe, unlike the US, where it has remained flat, he said. Meanwhile, better radar coverage has made it harder to move cocaine to the United States.

"Going that way, especially from South America, really gets you outside the majority of the security envelope for air traffic," said Decker, the criminology professor.

Venezuelan President Hugo Chavez's decision to sever ties with most US law enforcement agencies in 2005 has made it easier to bring cocaine to staging sites on the Venezuelan coast, said Vanda Felbab-Brown, a fellow at The Brookings Institution, a Washington think tank.

"The DEA is not present there, the Venezuelan military is making money off it, and much of the territory is just not controlled by the government," Felbab-Brown said.

The global economic slump has also idled hundreds of cargo jets, which can be bought cheaply. Ads on websites such as Planemart.com offer DC-8s for as low as $275 000.

The cases show the extraordinary lengths that traffickers are going to exploit the new air routes. The Valencia-Arbelaez gang used detailed spreadsheets to compute flight costs and distributed codebooks to conceal their plans.

Planning sessions were held in Denmark, Spain, Romania and a Best Western hotel in Manhattan. At one meeting the gang's leader, Jesus Eduardo Valencia-Arbalaez, sketched a map of West Africa showing points where the drugs would be delivered.

Fuel and pilots were paid for through wire transfers, suitcases filled with cash and, in one case, a bag of $356 000 in euros left at a hotel bar. The gang hired a Russian crew to move a newly acquired plane from Moldova to Romania, and then to Guinea.

Most of the cocaine was destined for Europe, but part of each shipment was supposed to go on to New York.

"I sold airplanes to these people so I knew what was going on," Manuel Silva-Jaramillo, an American aeronautical engineer, told a judge. "I knew that they were bringing the drugs to the United States."

The gang also discussed setting up a methamphetamine lab in Liberia and exporting the drug to Japan and the US.

The gang had access to a private airfield in Guinea, was considering buying its own airport and had sent a team to explore whether it could send direct flights from Bolivia to West Africa, Valencia-Arbelaez said in recorded conversations. A plane seized in Sierra Leone in July 2008 with 600kg of cocaine belonged to the group, the DEA says.

The European drug market was hugely profitable. Silva-Jaramillo claimed the gang had as much as $82m in euros stashed in Spain that it needed to launder, according to court documents.

Valencia-Arbelaez pleaded guilty to cocaine trafficking and was sentenced in July to 17 1/2 years in prison. Another conspirator, Javier Caro, received 3 1/2 years. Silva-Jaramillo and two other men have pleaded guilty and are awaiting sentencing.

Drug trafficking is especially dangerous to West Africa because of the corrupting effect it has on already weak governments, Felbab-Brown said.

In the Liberia case, traffickers offered bribes to Fumbah Sirleaf, the head of the Liberian security agency and son of the country's president. Sirleaf was secretly co-ordinating with the DEA.

The flights were to come from Venezuela and Panama. The ring had already sent aircraft into Liberia, Guinea and Guinea-Bissau, one of the traffickers was recorded saying.

The case has attracted attention in Russia because one of the defendants, Russian pilot Konstantin Yaroshenko, says he was tortured by Liberian police before being handed over to the DEA. He and the other five defendants have denied the charges against them.

The Russian foreign ministry accused the United States of "kidnapping" Yaroshenko and failing to tell the Russian government. Prime Minister Vladimir Putin called his arrest an example of the United States overstepping its bounds.

The DEA denies Yaroshenko was abused. The US Department of State said it mistakenly faxed Yaroshenko's arrest notice to the wrong embassy.