Japan and the United States signed an agreement Friday to expand their cooperation on a joint ballistic missile defense shield, the Japanese Foreign Ministry announced.

The pact, signed by Foreign Minister Taro Aso and U.S. Ambassador Thomas Schieffer, commits them to joint production of missiles to intercept incoming missiles, the ministry said in a statement.

The agreement had been previously negotiated and was not triggered by the current fears that North Korea is preparing to test a long-range missile. There has been speculation that the United States could try to intercept the missile if it is fired.

"The two countries have been negotiating for some time," said Foreign Ministry official Saori Nagahara.

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On Thursday a Navy ship intercepted a medium-range missile warhead above the earth's atmosphere off Hawaii, the military said.

The USS Shiloh detected a medium-range target after it was launched from the Pacific Missile Range Facility on Kauai, then fired a Standard Missile-3 interceptor.

The interceptor shot down the target warhead after it separated from its rocket booster, more than 100 miles above the Pacific Ocean and 250 miles northwest of Kauai, the Missile Defense Agency said in a statement.

The agency said the test had been scheduled for months and was not prompted by indications that North Korea was planning to test launch a long-range missile.

A similar test was conducted successfully in November of 2005, marking the first time a ship at sea shot down a multi-stage missile.

Chief Cabinet Secretary Shinzo Abe announced in December 2005 that Japan would jointly begin research on a missile shield with the U.S.

The pact allows the transfer of ballistic missile defense technology from Japan to the United States — a touchy issue in Japan, which has long adhered to a self-imposed ban on arms exports in line with the spirit of its pacifist constitution.

The new agreement updates a November 1983 pact on arms transfers and a December 2004 missile defense cooperation arrangement.

Nagahara said that the two governments have not decided when they will begin production of interceptor missiles, but that the development phase is expected to take about nine years.