Japan is unlikely to be able to join the U.S.-led Trans-Pacific Partnership trade negotiations by the next round in May, Singapore's chief TPP trade negotiator said Wednesday.

"I don't think we are looking at Japan specifically coming on board in Lima," Ng Bee Kim said at a news conference attended by all 11 negotiators at the end of the 16th round of TPP talks in Singapore.

She said countries can only join the full talks after bilateral or group consultations with other TPP countries to gain their approval, followed by all TPP members clearing domestic and legal hurdles.