While Japan's intensifying rivalry with China dominates the headlines, Tokyo's focus would be the ability to take out North Korean missile bases, said three Japanese officials involved in the process.

They said Tokyo was holding the informal, previously undisclosed talks with Washington about capabilities that would mark an enhancement of Japan's military might.

The talks on what Japan regards as a "strike capability" are preliminary and do not cover specific hardware at this stage, the Japanese officials told Reuters.

Defense experts say an offensive capability would require a change in Japan's purely defensive military doctrine, which could open the door to billions of dollars worth of offensive missile systems and other hardware. These could take various forms, such as submarine-fired cruise missiles similar to the U.S. Tomahawk.

U.S. officials said there were no formal discussions on the matter but did not rule out the possibility that informal contacts on the issue had taken place. One U.S. official said Japan had approached American officials informally last year about the matter.

Japan's military is already robust but is constrained by a pacifist Constitution. The Self Defense Forces have dozens of naval surface ships, 16 submarines and three helicopter carriers, with more vessels under construction. Japan is also buying 42 advanced F-35 stealth fighter jets.

Reshaping the military into a more assertive force is a core policy of Prime Minister Shinzo Abe. He has reversed a decade of military spending cuts, ended a ban on Japanese troops fighting abroad and eased curbs on arms exports.

RILING CHINA

China-Japan relations

Tokyo had dropped a request to discuss offensive capabilities during high-profile talks on revising guidelines for the U.S.-Japan security alliance which are expected to be finished by year-end, the Japanese officials said. Instead, the sensitive issue was "being discussed on a separate track", said one official with direct knowledge of the matter.

But any deal with Washington is years away and the obstacles are significant – from the costs to the heavily indebted Japanese government to concerns about ties with Asian neighbors such as China and sensitivities within the alliance itself.

The Japanese officials said their U.S. counterparts were cautious to the idea, partly because it could outrage China, which accuses Abe of reviving wartime militarism.

The officials declined to be identified because they were not authorized to discuss the closed-door deliberations. A Japanese Defense Ministry spokesman said he could not comment on negotiations with Washington.

Chinese Foreign Ministry spokeswoman Hua Chunying said Asian countries had a right to be concerned about any moves to strengthen Japan's military considering the country's past and recent "mistaken" words and actions about its history.

"We again urge Japan to earnestly reflect on and learn the lessons of history, respect the security concerns of countries in the region and go down the path of peaceful development," Hua told a daily news briefing in Beijing.

Japan would need U.S. backing for any shift in military doctrine because it would change the framework of the alliance, often described as America supplying the "sword" of forward-based troops and nuclear deterrence while Japan holds the defensive "shield".

Washington did not have a position on upgrading Japan's offensive capabilities, "in part because the Japanese have not developed a specific concept or come to us with a specific request", said another U.S. official.

"We're not there yet - and they're not there yet," the official said. "We're prepared to have that conversation when they're ready."

NORTH KOREAN MISSILES

North Korea lies less than 600 km (370 miles) from Japan at the closest point.

Japanese Prime Minister Shinzo Abe (R) reviews Japanese Self-Defence Forces' (SDF) troops during the annual SDF ceremony at Asaka Base in Asaka, near Tokyo, in this October 27, 2013 file photo. [Photo/Agencies]

Pyongyang, which regularly fires short-range rockets into the sea separating the Koreas from Japan, has improved its ballistic missile capabilities and conducted three nuclear weapons tests, its most recent in February 2013.

In April, North Korea said that in the event of war on the Korean Peninsula, Japan would be "consumed in nuclear flames".

Part of Japan's motivation for upgrading its capabilities is a nagging suspicion that the United States, with some 28,000 troops in South Korea as well as 38,000 in Japan, might hesitate to attack the North in a crisis, Japanese experts said.

U.S. forces might hold off in some situations, such as if South Korea wanted to prevent an escalation, said Narushige Michishita, a national security adviser to the Japanese government from 2004-2006.

"We might want to maintain some kind of limited strike capability in order to be able to initiate a strike, so that we can tell the Americans, 'unless you do the job for us, we will have to do it on our own,'" said Michishita, a security expert at the National Graduate Institute for Policy Studies in Tokyo.

Reflecting Japan's concerns, Abe told parliament in May 2013 that it was vital "not to give the mistaken impression that the American sword would not be used" in an emergency.

"At this moment is it really acceptable for Japan to have to plead with the U.S. to attack a missile threatening to attack Japan?" Abe said.

Under current security guidelines, in the event of a ballistic missile attack, "U.S. forces will provide Japan with necessary intelligence and consider, as necessary, the use of forces providing additional strike power".

SHROUDED IN EUPHEMISM

The informal discussions on offensive capabilities cover all options, from Japan continuing to rely completely on Washington to getting the full panoply of weaponry itself.

Japan would like to reach a conclusion in about five years, and then start acquiring hardware, one Japanese official said.

Tokyo had wanted the discussions included in the review of the Japan-U.S. Defense Cooperation Guidelines that are expected to cover areas such as logistical support and cybersecurity. Those talks, which formally kicked off last October, are the first in 17 years.

But the United States was keen to keep discussions on offensive capabilities separate to avoid riling China and South Korea, another Japanese official said. Beijing and Seoul each have territorial disputes with Tokyo and accuse Abe of failing to atone for Japan's wartime aggression.

Reflecting the sensitivities of the issue even in Japan, any talk of an upgraded offensive capability is shrouded in euphemism.

Itsunori Onodera, who stepped down last week as defense minister in a broad cabinet reshuffle, a year ago described it as "the capability to attack enemies' military bases and strategic bases for the sake of self-defense".

Defense guidelines compiled by the government in December watered this down to a "potential form of response capability to address the means of ballistic-missile launches and related facilities".