Talks on the ambitious Pacific free-trade treaty were extended again to Monday as they remained bogged down especially in differences over international dairy trade.

A US trade official said plans for a press conference expected to announce a deal on the 12-nation talks late Sunday were cancelled, though he gave no reason.

The official said nearly all outstanding issues had been dealt with, and that the negotiators were just completing details and reviewing the lengthy draft pact.

But various people close to the talks said New Zealand's demands for greater market access in a number of countries continued to slow progress in the final negotiations.

Trade ministers and other top officials from the 12 countries have been negotiating around the clock since Wednesday hoping to close the deal on the Trans-Pacific Partnership, which would be the world's largest free-trade zone.

Earlier in the day, Japan's Economy Minister Akira Amari said he was expecting a deal "in principle" to be announced Sunday afternoon.

Amari told Japanese journalists that there had been "major progress" Sunday morning and that a solution had been found to the main road block to a deal, how to protect developers of biologic drugs, which saw the United States and Australia sharply divided on the issue.

"We are making preparations now to announce a deal in principle this afternoon," he said, according to a translation of his remarks supplied by Japanese journalists.

The administration of President Barack Obama, the prime driver behind the TPP, wants it to create a foundation for "21st century trade rules," setting standards on trade, investment, data flows and intellectual property that eventually non-TPP members -- particularly China -- will have to accept.

Earlier there were signs that a compromise had been reached between the United States and Washington on one of the key final issues, how many years of patent protection should be given producers of biologics, a cutting-edge new class of drugs.  

The details of the compromise remained unclear; the US had been pushing eight years of protection while Australia insisted on five years.

According to the official, Peru and Chile, which had also opposed the US stance over biologics, had accepted the compromise.

But another hot issue of liberalizing trade in dairy products -- which would benefit New Zealand the most -- remained unresolved.

New Zealand, Australia and the United States have been pushing hard for protected markets like Japan, Mexico and Canada to open up.

But in each the issue remains a sensitive political issue, especially in Canada which will hold a national election on October 16.