Following his trip to the United States, Prime Minister John Key remains confident New Zealand can sign a free trade agreement (FTA) with the superpower.

But he does not know when.

After Mr Key travelled to Washington DC to attend the Nuclear Security Summit, he said New Zealand's relationship with the US was stronger than ever.

The US administration does want to expand US trading opportunities, he said today, but it would have to be a "two way" agreement.

"We wouldn't sign up to a FTA agreement that didn't provide what we thought was in the best interests of New Zealand (and neither would the US)," he told TVNZ's Breakfast.

"We can see the pockets of resistance, they're obvious for everyone to see, and they are the agriculture states in the United States of America and those senators have made their concerns clear."

Mr Key said those senators were trying to protect an industry that had "some protection about it".

"That's important to create jobs, just like it is for New Zealand, it is for America," Mr Key said.

"I think we can get there, yes, it'll be a hard fought road and we'll have to fight up against some quite strong lobby groups."