SERIOUS differences between the US and the EU over genetically modified crops and hormone-treated beef could well sink the free trade deal called the Transatlantic Trade and Investment Partnership (TTIP), says US Agriculture Secretary Tom Vilsack.

"This is not going to be an easy discussion. We know there are serious issues," Mr Vilsack told the EU Agricultural outlook conference.

"If we don't address these difficult, tricky issues, or decide not to deal with them because they are too hard, then in my view you are not going to have a TTIP agreement," he said.

"The agricultural interests in the United States, by themselves, are not politically powerful enough to get an agreement passed, but they are certainly powerful enough to stop agreement that's been negotiated," he said.

Mr Vilsack said three items had to be addressed: European opposition to US exports of beef and genetically modified organisms, and US opposition to the EU's system of geographical indications.

The United States says science shows US products from genetically modified crops to hormone-treated beef are safe, but such is met with scepticism in Europe.

"Geographical indicators" [GIs] are designed to ensure that only products from a specific region can carry a specific name such as champagne or Parmesan cheese. To the US, this is protectionism," he said.