The United States remains open to including China in a Pacific free-trade zone, Washington's top trade official said, less than a week after Japan asked to join the negotiations.

Acting US Trade Representative Demetrios Marantis said on Wednesday that it's up to China if the country wants to join the Trans-Pacific Partnership talks and satisfy the 11 nations now engaged that it could live up to the pact's requirements.

"Whether it's China, whether it's the Philippines, whether it's Thailand, it's incumbent upon those economies to be able to convince the other TPP partners that they are capable of meeting the high standards that we're negotiating," Marantis told reporters at a briefing on the US trade agenda for 2013.