US Secretary of State John Kerry on Sunday welcomed moves to admit Japan into a Pacific trade agreement, and said China's three-nation trade pact could act as a complement to the treaty.

After talks in Tokyo with Japanese Foreign Minister Fumio Kishida, Kerry said admitting Japan to the Trans-Pacific Partnership (TPP) would be a key step in the deal that would account for nearly 40 percent of the global economy.

"Clearly having Japan in the TTP would be an enormous economic benefit for all of us," Kerry told a press conference with Kishida.

Japan's entry would "be a critical mass for economic standards, and we believe it would help raise standards across the globe".

The United States gave Japan the green light on Friday to enter talks on the 11-nation Pacific trade agreement, despite opposition from some US manufacturers and labour groups and Japan's powerful farmers.

Under the TPP guidelines, all 11 nations engaged in the negotiations need to approve before Japan participates. Japan must still win over Australia, Canada, New Zealand and Peru.

Kerry said the US stress on economics was a large part of Washington's "rebalance" towards Asia, and hailed an agreement reached with Beijing on Saturday to strengthen their economic dialogue.

China's own free-trade agreement with Japan and South Korea could be "complementary" to the TPP and "ultimately if China wants to agree to standards and come up to that level, terrific", Kerry later told reporters travelling with him on a 10-day overseas trip.

"We're looking for partners, but we're looking for partners that want to raise the standards and meet those standards at the highest level."

Kerry will attend the next US-China Strategic and Economic Dialogue to be held in Washington July 8-12, in what the top US diplomat called an "upgraded" initiative with China.

Washington was hoping for "a much greater cooperative effort with China in the long run", he added.

Both Chinese and American businesses have run into problems operating in the other country. "So we need to resolve the standards between us and have a way of sitting down and getting quickly to those answers, rather than leave them to fester," Kerry said.