OMAHA, Neb. (AP) — Legal challenges could hinder plans to bring elephants from southern Africa to the Henry Doorly Zoo & Aquarium in Omaha.

The zoo and two others in Dallas and Wichita, Kansas, received federal permission this week to import 18 elephants from Big Game Parks, an independent nonprofit in Swaziland.

As part of the partnership, the three zoos will send about $450,000 over several years to Big Game Parks for black rhino conservation.

But animal-rights groups say they believe the exchange is an illegal commercial transaction. The Animal Legal Defense Fund, People for the Ethical Treatment of Animals and others have tried to delay or cancel the import since plans were first announced in 2014. The Animal Legal Defense Fund says it’s contemplating legal action.