Negotiators hope to wrap up talks on the Trans-Pacific Partnership (TPP) within months.

Canada is under increasing pressure from the United States and others to start dismantling its so-called supply management system of milk quotas and import tariffs that try to ensure steady prices for farmers.

"We have difficult choices in this one. We have some areas where obviously we see great advantages for Canada but others where there will be challenges," Harper said during a televised question-and-answer session in Saskatoon, a city in the western province of Saskatchewan.

"So we're obviously there at the table in what is a very important agreement doing our best to protect Canadian interests," he added.

Last month, the chairman of a U.S. congressional committee responsible for trade said Canada had to open its markets to farm imports under TPP. Any nation that could not meet the goals of the deal should drop out, he said.

The dairy lobby is powerful in Canada, where all three main political parties have vowed to maintain supply management. Canada is due to hold an election in October and if Harper moved to end the current system, he could suffer in the polls.

(Reporting by David Ljunggren; Editing by Peter Galloway and David Gregorio)