HONG KONG (BLOOMBERG) - With the Trump administration warning of a possible war with North Korea, US allies in Asia are sounding the alarm on another risk: a clash with China in the western Pacific.

China has recently accelerated air and naval excursions in sensitive areas near Japan and Taiwan, part of a long-standing quest to expand its military presence further from its shores into the Pacific Ocean. Leaders in Tokyo and Taipei have called on Beijing to back off while strengthening their defences.

Earlier this month, Japan observed for the first time a Chinese submarine entering the contiguous zone - 12 nautical miles to 24 nautical miles from shore - around disputed islets in the East China Sea.

That came shortly after Taiwanese President Tsai Ing-wen warned that China's increased military patrols around the island threatened to destabilise the region.

President Donald Trump's "America First" foreign policy has raised concern in Asia about the reliability of the US in helping to stave off Chinese pressure as it gains greater military and economic strength. China has a long-term goal of reuniting with Taiwan, and has territorial disputes with countries ranging from Japan to Vietnam to India.

"The unpredictability of the Trump administration encourages Tokyo and Taipei to do more for their own defence," said Chong Ja Ian, an associate professor with the National University of Singapore who specialises in Asia-Pacific relations.

"Unless resolved in such a way that all sides feel simultaneously assured, the actions can increase tensions in East Asia and raise the potential for some sort of incident."

While Trump's interactions with President Xi Jinping mostly focused on North Korea and trade during his first year in office, China's territorial claims may become more prominent going forward.

In a strategy document released last week, the US Defence Department cited China's military modernisation and expansion in the South China Sea as key threats to US power.

China has pushed back against that narrative, with its defence ministry over the weekend calling on the US to abandon a "Cold War" mindset. It blamed "other countries" for citing freedom of navigation concerns to undertake military activities in the South China Sea, where China has undertaken massive land reclamation to strengthen its claim to more than 80 per cent of the area.

On Saturday, China's foreign ministry said the country will take "necessary measures" to safeguard its sovereignty in the South China Sea after a US warship entered waters near the disputed Huangyan Island, also known as Scarborough Shoal.

'NEW NORMAL'

The Communist Party's official People's Daily on Monday (Jan 22) accused the US of destroying stability in the South China Sea, and threatened to "enhance and speed up" its military capacity in the waters in response.

China has also dismissed allegations that it is encroaching on Taiwan and Japan. Patrols around Taiwan by Chinese fighter jets, bombers and surveillance aircraft are the "new normal," Chinese Air Force spokesman Shen Jinke said last month. The Chinese submarine spotted near disputed islands in the East China Sea was monitoring the movements of two Japanese vessels, the foreign ministry said.

China's navy began sailing through the "First Island Chain" - including Japan, Taiwan and the Philippines - in 2009. The Air Force followed suit with regular patrols in 2015, and the frequency of flights has increased from "four times per year" then to "several times per month" in 2017, according to China's official Xinhua News Agency.

'CABBAGE STRATEGY'

Last year, Tsai said she would increase Taiwan's defence spending by at least 2 per cent each year. Priorities include new missiles, fighter aircraft and ballistic missile defences. The US continues to sell weapons to Taiwan and is obligated to defend the island under a 1979 law.

Japan's Cabinet last month approved a record defence budget of about 5.19 trillion yen (S$62 billion), the sixth straight annual rise in defence spending under Prime Minister Shinzo Abe.

While its missile defence purchases are primarily to deter North Korea, Defence Minister Itsunori Onodera said this month they could be used to stop other weapons.

Abe hosted Australian Prime Minister Malcolm Turnbull last week at a military base, part of efforts to strengthen a burgeoning four-way security arrangement that also includes the United States and India.

In an interview with the Australian Financial Review published on Saturday, Abe said the "Quad" grouping was not aimed at containing China even as he warned of instability in the region's waterways.

'NOT WORKING'

"There is an attempt to alter the present status in the East China Sea and the South China Sea," Abe told the publication. "So I think the security situation is becoming tougher these days."

China is employing a "cabbage strategy" in which it gradually surrounds a disputed area with multiple layers of security, according to June Teufel Dreyer, a University of Miami political science professor and author of Middle Kingdom And Empire Of The Rising Sun - a 2016 book on China-Japan ties.

"To the extent Taiwan and Japan can be said to have a strategy, it is to raise their deterrence capabilities to a level that keeps the situation stable," Dreyer said. "It's not working."