TOKYO (REUTERS) - Japan is likely to start considering acquiring the ability to launch pre-emptive military strikes in a planned update of its basic defence policies, the latest step away from the constraints of its pacifist constitution.

The expected proposal, which could sound alarm bells in China, is part of a review of Japan's defence policies undertaken by Prime Minister Shinzo Abe's government, an interim report on which could come as early as Friday.

The final conclusions of the review are due out by the end of the year.

The hawkish Abe took office in December for a rare second term, pledging to bolster the military to cope with what Japan sees as an increasingly threatening security environment including an assertive China and unpredictable North Korea.

Article 9 of Japan's constitution, drafted by US occupation forces after its defeat in World War Two, renounces the right to wage war and, if taken literally, rules out the very notion of a standing army. In reality, Japan's Self-Defense Forces are one of Asia's strongest militaries.

The Defence Ministry will call in the interim report for a study of how to "strengthen the ability to deter and respond to ballistic missiles", the Yomiuri newspaper and other media said on Thursday.

But in a sign of the sensitivity, the report will stop short of specifically mentioning the ability to hit enemy bases when the threat of attack is imminent, the Yomiuri newspaper said.

The ministry will also consider buying unmanned surveillance drones and creating a Marines force to protect remote islands, such as those at the core of a dispute with China, media said.

"The acquisition of offensive capability would be a fundamental change in our defence policy, a kind of philosophical change," said Marushige Michishita, a professor at the National Graduate Institute of Policy Studies.

Obtaining that capability, however, would take time, money and training, meaning any shift may be more rhetorical than real. "It's easier said than done," Mr Michishita added.

The updated guidelines could also touch on Mr Abe's moves towards lifting a self-imposed ban on exercising the right of collective self-defence, or helping an ally under attack, such as if North Korea launched an attack on the United States.

The defence review may also urge replacing a self-imposed ban on arms exports, that has been eased several times, making it easier for Japan's defence contractors to join international projects and reduce procurement costs.

Some experts stressed that the changes were evolutionary rather than a sudden transformation in Japan's defence posture.

"It's all part of a process of Japan edging away from the most restrictive interpretation of Article 9," said Richard Samuels, director of the MIT-Japan programme at the Massachusetts Institute of Technology.

Still, given Japan's strained ties with China over disputed isles and how to frame the narrative of Japan's wartime history, China is likely to react strongly to the proposals, which come after Mr Abe cemented his grip on power with a big win in a weekend election for parliament's upper house.

"No matter how Japan explains things, China will attack it pretty harshly," said Michael Green of the Washington-based Center for Strategic and International Studies.

Although China has been a nuclear power for decades and North Korea is developing nuclear arms, Japan says it has no intention of doing so.

Support has grown in Japan for a more robust military because of concern about China, but opposition also remains.

Japan last updated its National Defence Programme Guidelines in 2010 when the Democratic Party of Japan was in power.

Those changes shifted Japan away from defending areas to its north, a Cold War legacy, to a defence capability that could respond with more flexibility to incursions to the south, the site of the row with China over tiny, uninhabited islands.

Japan has for decades been stretching the limits of Article 9 and has long said it has the right to attack enemy bases overseas when the enemy's intention to attack Japan is evident, the threat is imminent and there are no other defence options.

But while previous administrations shied away from acquiring the hardware to do so, Mr Abe's Liberal Democratic Party in June urged the government to consider acquiring that capability.

Just what hardware might come under consideration is as yet unclear. And with a huge public debt, Japan may be in no position to afford the bill.

Japan already has a very limited attack capability with its F-2 and F-15 fighter jets, mid-air refuelling aircraft and Joint Direct Attack Munition guidance kit. Tokyo also plans to buy 42 Lockheed Martin's F-35 stealth fighters, with the first four due for delivery by March 2017.

Acquiring the ability to hit mobile missile launchers in North Korea - the most likely target - would require many more attack aircraft as well as intelligence capability for which Japan would most likely have to rely on the United States, Mr Michishita said. Cruise missiles might also be considered.

Obtaining the ability to strike missile bases in mainland China would be an even bigger stretch, experts said, requiring for example intercontinental missiles.

"It would cost lots of money, and take time, training and education to acquire a robust and meaningful capability," Mr Michishita said.