Zimbabwe's environment minister says US dentist Walter Palmer should be extradited for the "illegal" killing of Cecil the lion.

Oppah Muchinguri described Mr Palmer as a "foreign poacher" who financed an illegal hunt.

She said she understood the Prosecutor-General had started the process to have him extradited from the US.

"We are appealing to the responsible authorities for his extradition to Zimbabwe so that he can be held accountable for his illegal action," Mrs Muchinguri told a news conference.

The US has an extradition agreement with Zimbabwe, so in theory he could be sent there to face the legal system if charges are filed.

Mr Palmer, from Minnesota, has said in a statement that he regretted the killing, but added that he had relied on his guides to ensure the hunt was legal.

The White House has said it will review a petition urging the Obama administration to send Mr Palmer to Zimbabwe to face justice.

More than 160,000 signatures have been gathered for the bid so far.

The We the People petition urges Secretary of State John Kerry and Attorney General Loretta Lynch to extradite the trophy hunter.

Any petition to the White House that attracts over 100,000 names within 30 days must receive an official response.

White House spokesman Josh Earnest said it is up to the Justice Department to respond to an extradition order.

The US embassy said it was not aware of any extradition request.

The US Fish and Wildlife Service announced on Thursday it had opened an inquiry into the matter.

"We're investigating the killing of #CecilTheLion. Will go where facts lead," the agency said on Twitter.

"We ask Dr Palmer or his rep to contact USFWS immediately."

But Palmer, 55, who reportedly paid park guides $50,000 (around €45,700) to kill the lion during a hunting trip this month, remains in hiding amid global uproar.

Protests

Protesters have gathered outside his suburban Minneapolis dental practice holding signs, including one that said: "Let the hunter be hunted!"

Amid reported death threats, police have stepped up patrols around Palmer's office, which has been temporarily closed.

Cecil's death was cited at the United Nations on Thursday as its General Assembly passed a resolution calling on all countries to step up efforts to tackle illicit wildlife poaching and trafficking.

The measure was adopted followed a two-year campaign by Germany and Gabon.

Germany's UN Ambassador Harald Braun told reporters: "Like most people in the world we are outraged at what happened to this poor lion."

Zimbabwean authorities said the men used a dead animal to lure Cecil from a protected area at Hwange National Park.

The big cat is said to have been wounded with a crossbow, tracked for 40 hours and then shot with a rifle.

The 13-year-old lion - who was being studied by Oxford University researchers - was decapitated and skinned.

Palmer has told his local newspaper, the Star Tribune, that he "deeply regrets" killing the animal.

But he said he had believed the hunt was legal, blaming his professional guides.

Two Zimbabwean men allegedly hired by Palmer have appeared in court on charges of poaching.

They face up to 15 years in prison if convicted.

Palmer has a poaching conviction for the 2008 killing of a black bear in western Wisconsin.

Some in Zimbabwe are miffed by the fuss over Cecil's death.

Harare resident Eunice Vhunise told the Associated Press: "We have water shortages, no electricity and no jobs - yet people are making noise about a lion?"

Zimbabwe's information minister, meanwhile, seemed clueless about the furore.

Asked about Cecil's demise, Prisca Mupfumira said: "What lion?"