A Zimbabwean conservation group on Friday said it was investigating whether a hunting trip by US property magnate Donald Trump's sons was legal, after photos of their trophies sparked outrage online.

Trump's sons Donald Junior and Eric made a hunting expedition in Zimbabwe in August 2010. Their pictures went viral on social media this month, showing them posing next to carcasses of a leopard, an elephant, a crocodile and an array of other animals.

The images drew condemnation on Facebook and Twitter, where their hunting trip was slammed as unethical. Donald Trump Jr has insisted on Twitter that he did nothing wrong.

Johnny Rodriguez, chairman of the Zimbabwe Conservation Task Force told AFP that most professional hunt guides register with his organisation or with the government-run Zimbabwe Parks and Wildlife Management Authority.

But he said his organisation does not have a record of the Trump brothers' hunting trip.

"We have people on the ground investigating. We want to find out if the hunting was ethical," he said.

"There is a lot of corruption and unethical conduct going on in the safaris. We want to find the truth and where the money generated from the hunting went to."

"We have been told that the Trump brothers were saying they gave game meat to locals who were starving, that's an insult to local people," he said.

"These people are wealthy. If they wanted to help the people of Zimbabwe, why didn't they build schools or something like that, than to say that they gave meat to starving people. The area they are said to have hunted is Matetsi and there are no people who live there."

The trip was organised by Pretoria-based Hunting Legends International, which specialises in big game hunts for the wealthy.

The company has insisted that the hunt near the world-famous Victoria Falls was completely legal, and has kept pictures of the Trumps with their kills on its website.

"The entire hunt was done strictly according to the laws of the Department of Nature Conservation in Zimbabwe and, as is custom in these government-controlled areas, a staff member of the department escorted the hunt at all times," it said in a statement on its website.

"The hunt was entirely legal and ethically conducted as prescribed by the industry norms and regulations," it said. "None of the animals hunted were on the endangered species list and are plentiful in the area the hunt was conducted on."

But the outrage over the hunting trip has already cost Donald Trump one advertiser from his US reality television show "The Apprentice", according to US media.

The pictures show Donald Junior holding a knife and a severed elephant tail, and sitting with a rifle next to a slain Cape buffalo. The brothers are also seen holding a leopard carcass, and standing next to a crocodile hanging from a tree.

"Donald Trump Jr. your're loser, ditto head. i'm a hunter. i hunt for meat, not horns. you can't eat horns," read one Twitter post.

"Men who hunt are not men," read another.

"Someone needs to stop these fools, maybe even hunt them," another said.

Donald Trump Jr has insisted the trip was legitimate.

"In the area I was in, the the wildlife board wanted to decrease the numbers for the benefit of the herd. Like liberal deer limits," he tweeted. "I am also not going to apologize because some eco nuts want me 2."