Japan plans to hold bilateral talks on the elimination of tariffs with six countries through mid-September under the Trans-Pacific Partnership initiative to make up for its late start in the negotiation process, government sources said.

Officials aim to report progress on the talks with Australia, Canada, Chile, Peru, the United States and Vietnam at a four-day meeting of chief TPP negotiators starting Sept. 18 in Washington, the sources said.

During the nine-day TPP round that ended Friday in Brunei, Japan held talks with Australia, Canada, the U.S. and Vietnam among the six but was unable launch full-fledged negotiations by exchanging lists of proposals on tariff-free items.

It did exchange such lists with Brunei, Malaysia, Mexico, New Zealand and Singapore.

By mid-September, Japan may also have a second round of one-on-one talks with these countries.

Koji Tsuruoka, Japan's chief negotiator, said at a news conference Friday that he had been told by countries that have already held the first round of talks with Japan that the proposals still have room for improvement.

"We will come up with additional proposals," Tsuruoka said.

The 12 countries negotiating the TPP agreed in late July to put 95 percent of their respective tariff-free items on the negotiating table by Sept. 20 as part of a four-stage timetable for reaching a basic agreement by the end of October.

The deadline has apparently led Japan to speed up its negotiation process.

The 12 countries — Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States and Vietnam — are seeking to conclude an overall TPP deal by the end of this year.

Together they account for nearly 40 percent of global economic output and about a third of world trade.