Prince William has met President Barack Obama at the White House as part of a three-day visit.
The talks were held in the Oval Office, in advance of delivering a speech to the World Bank on wildlife crime.
During the speech, Prince William described the illegal wildlife trade as “one of the most insidious forms of corruption and criminality in the world today”.
Meanwhile, Kate Middleton visited a New York children’s centre.
Prince William and Kate Middleton arrived in New York on Sunday night, with the prince travelling to Washington alone on Monday.
He added that he expects a “busy year” in 2015, when Kate Middleton is due to give birth to the royal couple’s second child.
Barack Obama said the prince’s work to protect endangered wildlife was “very important”.
Later, in his speech at the World Bank, Prince William said: “In my view, one of the most insidious forms of corruption and criminality in the world today is the illegal wildlife trade.”
He attacked those who “loot our planet to feed mankind’s ignorant craving for exotic pets, trinkets, cures and ornaments derived from the world’s vanishing and irreplaceable species”.
Prince William founded the umbrella organization United for Wildlife to try to preserve the planet’s most endangered animals and habitats.
The Duke of Cambridge has been the royal patron of Tusk since 2005. The charity supports projects in 17 African countries, aimed at protecting wildlife and alleviating poverty.
Before starting at St Andrews University, Prince William went on a gap year, some of which was spent in Africa learning about its wildlife and game conservation.
Paying tribute to his father, Prince Charles, and his grandfather, the Duke of Edinburgh, he said: “They helped to bring about a revolution in attitudes towards our natural environment.”
He added: “From them, I learned that our relation to nature and wildlife goes to the heart of our identity as human beings. From our sheer survival, to our appreciation of beauty and our connection to all other living things.
“Seen in this light, the extinction of any of the world’s species of animals is a loss to all humanity.”
Prince William said wildlife crime “goes to the heart of our security”, “distorts economic development” and “fuels sources of conflict”.
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