Three American zoos have pulled off an audacious clandestine operation to fly a group of elephants out of Swaziland despite a legal challenge to block the transfer, enraging conservationists who claim that removing elephants from the wild for display in zoos is cruel and outdated.

The 18 elephants – three males and 15 females ranging in age from six to 25 – were sedated, loaded onto crates and placed on a large cargo plane that arrived in Swaziland at around 7.30am local time on Tuesday. Pictures taken at the scene show several large crates, ostensibly for elephants, being picked up by a crane and placed onto trucks.

The elephants will be split among three zoos – the Dallas zoo, Sedgwick County zoo in Kansas and Henry Doorly zoo in Nebraska – where they will be put on exhibit and used for breeding purposes.

The unannounced move, which was previously scheduled for May, has stunned an animal welfare group that was due to argue against the transfer in a US federal court on 17 March. The group, Friends of Animals, has now admitted defeat in its bid to prevent what it calls the “stolen 18” from being taken from the wild and placed into zoos.

Friends of Animals filed for an emergency injunction to stop the transfer, argued via a frantic teleconference with US federal court judge John Bates. Bates’s ruling notes that the short timeframe meant he was “not able to definitively resolve the issue”. But he sided with the zoos, which pointed out that the elephants had already been drugged and that leaving them in the small African nation would have put them at risk.

Michael Harris, legal director of Friends of Animals, said the last of the elephants were transferred from temporary holding areas to the plane on Wednesday, ahead of their 9,000-mile journey to the US.

“This is blatantly underhanded, I’ve never seen anything like it in 20 years doing this,” he said. “This is a devious and dramatic way to avoid the court hearing on 17 March. I certainly think the legal fight to save them is over. But the fight to make them martyrs is just beginning – they really are the stolen 18.

“The drugging and crating will be very traumatic for the elephants and will live with them for a long time. The majority of these elephants are under 12 years old, which means they will spend the next 50 years feeling like captives. They will suffer in captivity, there is no doubt of that.”

The US Fish and Wildlife Service has granted a permit to the zoos to import the elephants, amid concerns that they will be culled due to the intense drought that has gripped southern Africa. The zoos have said the removal of the elephants from Swaziland’s Hlane national park will relieve pressure on other wildlife, such as rhinos, and has called opposition to the transfer “ideological”.

In a joint statement, the three zoos said they had to rapidly remove the elephants for their own wellbeing due to the deteriorating situation in Swaziland.

“As food supply became scarcer, urgency increased to relocate the elephants to homes where they could be well cared for,” the zoos said.

“The attempt by activists to further delay the relocation only jeopardized the animals. The elephants would have been killed if not relocated. Scarcity of food and water and the risk of malnutrition are creating the most significant health threat to people and animals throughout the region. For the sake of the elephants, we moved quickly to relocate these animals to safe homes with a secure future.”

The Association of Zoos and Aquariums, the accrediting body for zoos, said it supports the removal of the elephants. A spokesman said the three zoos have “taken every precaution” to safeguard the animals.

But activists have argued that the elephants should be moved elsewhere in Africa, claiming that Swaziland’s king has prevented them from migrating and finding food. Non-profit group Groupelephant.com said it was willing to relocate them to South Africa, calling the US transfer “purely a money-making transaction for Swaziland the zoos involved”.

The drought in southern Africa, fueled by the El Niño climatic event, is a further burden to bear for the continent’s dwindling population of elephants. Recent data released by the UN shows that more African elephants are being killed for ivory than are being born, meaning that the overall population is in decline. It’s estimated that an elephant is killed by poachers roughly every 20 minutes.

Conservationists maintain there are signs of hope for the species, despite the loss of 20,000 elephants to poaching in 2015. Poaching rates peaked in 2011 and have declined markedly in countries such as Kenya.

But the species remains under severe pressure, with Barack Obama voicing fears earlier this month that elephants could vanish in the wild during his lifetime. The US president said such an event would be an “unpardonable loss for humanity and the natural world”.