WASHINGTON - The United States on Friday cautiously welcomed Japan's interest in joining negotiations on a US-led Pacific free trade pact, emphasising that Tokyo must demonstrate its willingness to tackle longstanding trade barriers to US goods and services.

"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," Mr 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 US cars.

Mr Marantis said the administration "will continue to consult with Congress and stakeholders" as it considers whether to support Japan's entry into the talks.