WASHINGTON -- Japan remains fully committed to building a linchpin multibillion-dollar missile interceptor with the United States, the head of the US Missile Defense Agency told Congress, even as US-Japanese ties adjust to a new era.

"They have indicated that they are in full support and their commitments are solid," he told the Senate Appropriations Defense subcommittee, referring to the Standard Missile-3 upgrade program in its fifth year of development.

Published reports from Japan have said the coalition government of Prime Minister Yukio Hatoyama that took power in September plans to reduce missile-defense spending.

Japan already has spent just over $1 billion to help build a more capable SM-3 version, said Richard Lehner, a US Missile Defense Agency spokesman. It is being co-developed with Waltham, Massachusetts-based Raytheon Co, the world's biggest missile maker.

The new version, dubbed SM-3 Block IIA, is key to US plans to be able to defend all of NATO's European territory from a perceived Iranian ballistic missile threat as soon as about 2018.

It is designed to improve the antimissile's velocity, range and ability to discriminate among a ballistic missile target and any decoys, and would be deployed on land as well as at sea. A follow-on version, called Block IIB, with yet higher velocity, is intended to help protect the US East Coast from potential long-range Iranian missiles by about 2020.

O'Reilly said the United States and the Hatoyama government had identified all steps necessary to successfully integrate the upgraded Block IIA SM-3 interceptor.

Its first flight test should be in 2014 and the first intercept test in 2015, he said.

"Within the next year, we will begin our discussions on production arrangements between the United States and Japan," he said.

Since the Democratic Party of Japan's victory, bilateral tensions have arisen over the desire of some Hatoyama government members to change a 2006 US-Japan deal to relocate a controversial US Marine air station to a less densely populated spot on Okinawa.

The SM-3 co-development program represents "not only an area of significant technical cooperation but also the basis for enhanced operational cooperation to strengthen regional security," Bradley Roberts, a deputy assistant secretary of defense, told a House of Representatives Armed Services subcommittee on April 15.