Nina Fedoroff told the BBC One Planet programme that humans had exceeded the Earth's "limits of sustainability".

Dr Fedoroff has been the science and technology advisor to the US secretary of state since 2007, initially working with Condoleezza Rice.

Under the new Obama administration, she now advises Hillary Clinton.

"We need to continue to decrease the growth rate of the global population; the planet can't support many more people," Dr Fedoroff said, stressing the need for humans to become much better at managing "wild lands", and in particular water supplies.

Pressed on whether she thought the world population was simply too high, Dr Fedoroff replied: "There are probably already too many people on the planet."

GM Foods 'needed'

A National Medal of Science laureate (America's highest science award), the professor of molecular biology believes part of that better land management must include the use of genetically modified foods.

"We have six-and-a-half-billion people on the planet, going rapidly towards seven.

"We're going to need a lot of inventiveness about how we use water and grow crops," she told the BBC.

"We accept exactly the same technology (as GM food) in medicine, and yet in producing food we want to go back to the 19th Century."

Dr Fedoroff, who wrote a book about GM Foods in 2004, believes critics of genetically modified maize, corn and rice are living in bygone times.

"We wouldn't think of going to our doctor and saying 'Treat me the way doctors treated people in the 19th Century', and yet that's what we're demanding in food production."

In a wide ranging interview, Dr Fedoroff was asked if the US accepted its responsibility to reduce emissions of carbon dioxide, the gas thought to be driving human-induced climate change. "Yes, and going forward, we just have to be more realistic about our contribution and decrease it - and I think you'll see that happening."

And asked if America would sign up to legally binding targets on carbon emissions - something the world's biggest economy has been reluctant to do in the past - the professor was equally clear. "I think we'll have to do that eventually - and the sooner the better."