Two elephants are seen at Sedgwick County Zoo in Wichita, Kansas, in 2012. The zoo is one of three in the US to get some of the 15 elephants being airlifted from Swaziland.

Nearly half the elephant population of the tiny, landlocked African country of Swaziland are en route to US zoos in Omaha, Wichita and Dallas.

Depending on who's describing it, the airlift operation will either save the elephants' lives - and those of threatened rhinoceroses - or condemn the pachyderms to suffer post-traumatic stress disorder for the financial benefit of zoos and a corrupt kingdom.

The transfer of the elephants, on Wednesday (US time) came despite a lawsuit filed last month by the advocacy group Friends of Animals, which challenged the US Fish and Wildlife Service's issuance in January of a permit for the import. A hearing had been set for March 17, but a US district court judge denied the organisation's last-minute request on Tuesday for a temporary restraining order to halt the import, noting that the 18 animals had been sedated in preparation for the planned flight. Friends of Animals called the decision to transfer the animals a "devious" move intended to avoid the court date.

At issue are some of the thorniest questions in wildlife conservation - how to rescue threatened species that sometimes compete for resources and whether confinement in zoos is an act of cruelty or conservation.

Swaziland, like the rest of Southern Africa, is experiencing a severe drought that has caused a scarcity of food sources for wildlife, and the nonprofit that manages its parks said it would cull the elephants to offset what it says is an overpopulation that is putting pressure on critically endangered black rhinoceroses. The elephants had been moved last July from national parks to temporary holding areas, where they were fed hay trucked in from South Africa and partially paid for by the three zoos: the Dallas Zoo, Omaha's Henry Doorly Zoo and the Sedgewick County Zoo in Wichita, Kansas.

A partnership between the zoos and officials in Swaziland, called Room for Rhinos, in September hailed the transfer as a "necessary" decision that would "prevent further degradation of the landscape and in order to make room for critically endangered rhinos". Moving them elsewhere in Africa, it said, was "unrealistic due to issues related to excessive poaching, loss of habitat and elephant-human conflicts." The import, they said, would help educate American zoo-goers about the threats to both elephants and rhinos. About 20 other elephants will remain behind in Swaziland.

But conservationists, scientists and animal welfare groups have criticised the transfer, which is the first import of Swazi elephants to the United States since 2003. They argued that elephants, which can roam dozens of kilometres each day in the wild, are prone to stress from transport and separation from family members. Friends of Animals cited studies finding the animals suffer health problems and die at much earlier ages in captivity, and in October, 75 elephant experts released an open letter questioning the conservation value of the move.

"In reality, the entire population of fewer than 35 elephants occupies only small fenced portions of the reserves and poses no considerable threat to other wildlife; no evidence has been presented to show significant habitat competition with rhinos," they said.

Critics have also pointed to the zoos' pledge to spend US$450,000 (NZ$676,010) over the next five years to support Swaziland's rhino conservation programmes, saying it amounts to a payment that could end up in the pockets of Swaziland's Big Game Parks system and its monarchy. The zoos, they say, will also benefit financially from the import.

"Both parties know that the world increasingly sees the purchase and importation of African big game as morally repugnant, even if it's not out-and-out illegal," the Animal Legal Defense Fund said in November. "Thus, the transaction is shaded as something other than a direct sale."

The Omaha and Wichita zoos have another reason to import elephants: The Association of Zoos and Aquariums, which accredits zoos in the Untied States, now requires that those with elephants have at least three females, two males or three elephants of mixed gender by September 2016. Both of those zoos have only one elephant.

In its lawsuit, Friends of Animals argues that Fish and Wildlife should not have granted a permit without considering the emotional and physical well-being of the animals, as mandated by the National Environmental Policy Act. Tim Van Norman, head of the permits branch at Fish and Wildlife, told National Geographic last month that the agency was required to determine that the move would not be detrimental to the species, as opposed to individual animals, and that the elephants would be suitably housed and cared for by experts. Those conditions were met, he said.

The 18 elephants include three males and 15 females ranging in age from 6 to 25. Each zoo is set to receive six elephants. Those in Dallas will live in a six-acre (2.42ha) enclosure with four other elephants, and those that alight in Kansas will call a five-acre outdoor and 18,000-square-foot (1672sqm) indoor space home. The Omaha zoo is constructing a US$73 million, 28-acre "African Grasslands" exhibit that includes a 29,000-square-foot "elephant family quarters" that it says will feature the largest herd room in North America.