Members of the Trans-Pacific Partnership negotiations are considering a plan to phase out import tariffs over a maximum period of 30 years, negotiation sources said Monday.

The plan could make it easier for the countries negotiating the free trade deal to reach an agreement and uphold its principle of eliminating tariffs on all items, although the time frame may raise concerns about how sincere they are about achieving a high level of trade liberalization.

The plan has been proposed by some of the developing economies in the TPP, including Vietnam, who believe their domestic industries still need time to become internationally competitive, the sources said.

The length of the proposed phaseout period compares with the 10-year period usually adopted by other free trade agreements.

During the ministerial meeting of the 12 TPP countries from last Thursday to Sunday on the Indonesian island of Bali, the members agreed to uphold its basic rule of eliminating all tariffs.

Afterward, a member of Japan's ruling Liberal Democratic Party said it will start reviewing the possibility of eliminating tariffs on its most sensitive items, such as rice, that were previously considered untouchable. The government then tried to downplay the issue.

Japan faces strong domestic pressure from farmers to retain tariffs on imports of rice, wheat, beef and pork, dairy products and sugar to protect domestic agriculture.

The 12 TPP members — Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States and Vietnam — are holding a series of meetings on the margins of the Asia-Pacific Economic Partnership forum and are scheduled to announce a broad agreement at the end of a TPP summit on Tuesday.