"Since early last year, the United States has been engaged with Japan in bilateral TPP (Trans-Pacific Partnership) consultations on issues of concern with respect to the automotive and insurance sectors and other non-tariff measures," acting U.S. Trade Representative Demetrios Marantis said in a statement.
"While we continue to make progress in these consultations, important work remains to be done," Marantis said.
Japanese Prime Minister Shinzo Abe announced earlier on Friday that Japan will formally seek to join talks on the Trans-Pacific Partnership, a proposed free trade agreement between the United States and ten other countries in the Asia-Pacific.
Negotiators hope to conclude that pact by the end of year, and possibly as early as the annual Asia Pacific Economic Cooperation leaders meeting in October.
But Japan's entry, if approved by the current TPP members, would make it even more challenging to meet that timetable.
It also creates anxiety for Detroit-based automakers who fear losing more sales to Japanese imports.
Ford has been especially vocal in opposing Japan's entry into the talks until Tokyo demonstrably opens its market to more U.S. cars.
Marantis said the administration "will continue to consult with Congress and stakeholders" as it considers whether to support Japan's entry into the talks.
The chairman of the House of Representatives Ways and Means Committee, which oversees trade issues, also responded cautiously.
"I remain concerned that Japan has not yet provided adequate assurances that it is fully committed to resolving the outstanding barriers to trade between the United States and Japan, especially as it relates to our auto exports and insurance, which is essential to receiving my support for Japan to join the negotiations," Representative Dave Camp, a Michigan Republican, said in a statement.
Camp also said he did not want Japan's interest in joining the talks to "unravel the significant progress we have already made in the TPP negotiations or delay the conclusion of TPP by the APEC Summit in October."
Dozens of Democratic lawmakers on Thursday expressed concern about Japan's entry in the negotiations in a letter to President Barack Obama.
Although Japan has no tariffs on U.S. autos, Ford and many U.S. lawmakers contend the country maintains a web of regulatory and other non-tariff barriers to keep out autos.
In their letter to Obama, the lawmakers urged the United States keep its 2.5 percent tariff on Japanese autos and 25 percent tariff on Japanese trucks if Japan enters the TPP. (Reuters)