The US government is suing to force an auction house to return the fossil of a dinosaur that roamed Mongolia 70 million years ago to the Asian country.

The nearly complete tyrannosaurus bataar is believed to have been unearthed between 1995 and 2005, and is one of several specimens first discovered in the Gobi desert in 1946.

Despite a Mongolian law declaring fossils property of the state and banning their export, the skeleton weighing a tonne and measuring 24 feet (7,31 meters) long and eight feet (2,43 meters) tall arrived in Gainesville, Florida in March 2010.

It was sold at a New York auction on May 20 for $1.05 million by Heritage Auctions, a defendant in the lawsuit.

According to documents filed Monday in US District Court in New York, it is alleged to have been illegally imported from Britain by smugglers who made false claims about what it was and its value to get it past US customs.

US authorities are now demanding that it be handed over to the United States so that it can be returned to Mongolia.

The government of Mongolia had obtained an injunction to prevent the sale of the skeleton from a judge in Texas where the auction house is based, but it went ahead anyway,

"We auctioned the Tyrannosaurus bataar conditionally, subject to future court rulings, so this matter is now in the hands of lawyers and politicians," Jim Halperin, co-chairman of Heritage Auctions said.

US Attorney Preet Bharara said in a statement the skeletal remains are "of tremendous cultural and historic significance to the people of Mongolia, and provide a connection to the country's prehistoric past.

"When the skeleton was allegedly looted, a piece of the country's natural history was stolen with it, and we look forward to returning it to its rightful place," he said.

The news release also included a quote from Mongolia's President Tsakhia Elbegdorj, saying he was thankful for US efforts to recover the skeleton, calling it "an important piece of the cultural heritage of the Mongolian people."

A June 5 examination by three bataars experts has concluded that the tyrannosaurus bataar skeleton almost certainly originated in the Nemegt Basin in Mongolia.

But according to Heritage, the consignor purchased the fossil in "good faith, then spent a year of his life and considerable expense identifying, restoring, mounting and preparing what had previously been a much less valuable matrix of unassembled, underlying bones."

A statement on the day of the auction said it marked "the first time a fully prepared Tyrannosaur has been made available at public auction."