U.S. Defense Secretary Ashton Carter, left, talks with Japan's Prime Minister Shinzo Abe at the start of their meeting at Abe's official residence in Tokyo, Wednesday, April 8, 2015. The first revision of the U.S.-Japan Defense Guidelines in 17 years will "transform" the bilateral alliance, Carter said Wednesday. (AP Photo/Franck Robichon, Pool) (Franck Robichon/AP)

The United States and Japan are close to concluding a set of bilateral defense rules that if finalized would give Japan’s military new powers to act when U.S. forces are threatened by a third country, U.S. officials said Wednesday.

Defense Secretary Ashton Carter, speaking during a visit to Tokyo, said the revision of the “defense guidelines” would transform U.S. military ties with Japan, which is grappling with a missile threat from North Korea and China’s moves to assert control of areas off its coast.

Under a previous bilateral arrangement, Japanese forces could protect the U.S. military only if it was operating in Japan’s direct defense in areas close to the country. U.S. officials say the new rules, once given final approval, would broaden the geographic scope and, significantly, allow Japan to respond to an attack on the U.S. military even if the American forces are not acting in defense of Japan at the time.

A senior defense official, speaking on the condition of anonymity to discuss ongoing negotiations, called the new rules “a big, big deal.” He said they were also intended to empower Japan to use its missile defense systems to protect U.S. military assets under a greater range of circumstances.

“With missile defense, the more radars you have and the more shots you have, the higher the probability of a kill is,” he said, referring to the likelihood that a missile could be intercepted.

The revised rules, which are expected to be unveiled later this month ahead of Prime Minister Shinzo Abe’s visit to Washington, mark an evolution in Tokyo’s military ties with the United States. Under a 1960 treaty, the United States is committed to protecting Japan from external aggression. About 50,000 U.S. troops are stationed in Japan.

The changes take place as the conservative Abe continues his effort to overhaul how Japan uses its military power, which has been tightly restricted since World War II. Abe is now seeking passage of legislation that would provide greater power to the military, known as the Self-Defense Forces, to protect Japan and its allies, and to take part in limited military cooperation overseas.

Ely Ratner, a senior fellow at the Center for New American Security, a Washington think tank, said the changes will create “a more equal alliance of mutual defense, rather than simply the defense of Japan.”

Eventually, “this has the potential to open up a variety of new defense activities for Japan in Asia and beyond that are currently forbidden,” he said.

Many in Japan are increasingly alarmed by Beijing’s dramatic military buildup and its efforts to project Chinese power in areas of the South China Sea and East China Sea, such as the disputed islands known to the Japanese as the Senkakus and to the Chinese as the Diaoyu.

The Obama administration, also worried by China’s military rise, has welcomed Abe’s moves. U.S. officials say they hope greater military flexibility will provide Japan extra leverage to resolve disputes with China through diplomatic means.

Defense Minister Gen Nakatani said the revised rules would also bolster joint efforts on cyber issues and in space.

Speaking to reporters after talks with the Japanese minister, Carter addressed the conflict in Yemen, where Iranian-backed Houthi rebels recently toppled the government. He said the crisis there has provided an opening for al-Qaeda in the Arabian Peninsula (AQAP), the potent militant group that seeks to strike the United States.

“AQAP is another [group] that has seized the opportunity of the disorder there and the collapse of the central government,” Carter said “We see them making . . . gains on the ground there as they try to take territory.”

In recent days, the group’s fighters have seized a border post near Saudi Arabia and launched an attack in the Yemeni port of Mukalla.

The administration is backing a coalition of Sunni Arab nations, led by Saudi Arabia, that is using air power in an attempt to halt the Shiite rebels’ advance. On Tuesday, a top State Department official said the United States was accelerating arms supplies and stepping up intelligence sharing with Saudi Arabia.

But top U.S. military officials have said that their ability to counter AQAP is diminished, even though American drones continue to fly over Yemen. As fighting has intensified, the United States has pulled out its remaining troops from Yemen, dealing a blow to the counterterrorism effort there.

“That doesn’t mean that we don’t continue to take steps to protect ourselves,” Carter said. “We have to do it in a different way, but we do and we are.”

Anna Fifield contributed to this report.

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