Negotiations for a regional free trade deal, which would include the United States, have hit a snag which could cost New Zealand dairy farmers free access to the world's biggest market.

The US wants existing trade agreements between the eight countries negotiating the Trans Pacific Partnership left in place.

The Dairy Companies Association says New Zealand farmers want better access to US markets than Australians got in their deal in 2003, but Association Executive Director Simon Tucker says the Americans prefer bilateral talks.

Trade Minister Tim Groser says that could make it harder for New Zealand to better a deal Australia signed with the United States seven years ago.

The Trans Pacific Partnership had been close to wrapping up but now the United States wants to negotiate separate new deals within it with New Zealand and Vietnam.