Australia and Japan signed contracts on Saturday to launch a $7 billion deal to supply Canberra with warships, marking Tokyo's biggest military sale since the end of a military export ban in 2014.

The move reflects a steady deepening of bilateral defense cooperation, with experts warning that such collaboration could evolve into a more exclusive "quasi-alliance", potentially contributing to bloc confrontation in the Asia-Pacific region.

Australian Defense Minister Richard Marles and his Japanese counterpart Shinjiro Koizumi, signed a memorandum "reaffirming the Australian and Japanese governments' shared commitment to the successful delivery" of the warships, Marles said in a statement.

According to the memorandum, Mitsubishi Heavy Industries will build the first three Mogami-class frigates in Japan. Australia plans to build another eight at a shipyard in Western Australia.

Australia announced in August last year that the Japanese bid had won the contract to provide Australia's next-generation general-purpose frigates over Germany's MEKO A-200 from ThyssenKrupp Marine Systems.

The deal gave a major boost to Japan's defense industry after it lost out on Australia's submarine contract to a French company in 2016.

Chinese Foreign Ministry spokesman Guo Jiakun on Monday urged Japan to draw lessons from history, abide by its international obligations and stay committed to the path of peaceful development following large-scale public demonstrations in Tokyo against the Japanese government's push to revise the constitution.

Preventing the resurgence of militarism is Japan's bounden obligation, and it is also the firm will of the international community, including China, Guo stressed.

Marles said the Japanese frigates were a major step toward delivering Australia a larger and more lethal surface combat fleet. The first of the Mogami-class frigates is due to arrive in Australia in 2029.

Japan has been accelerating its military buildup while expanding its defense ties beyond its only treaty ally, the United States. It now considers Australia to be a "semi-ally".

Koizumi said the introduction of Japanese vessels into the Australian navy meant "a major step is finally being taken to elevate our bilateral defense cooperation to a greater height".

He said Japan remained Australia's "indispensable partner" in a new Australian defense strategy announced recently.

On Thursday, Australia declared it will increase military spending by $38.1 billion over the next 10 years under the 2026 national defense strategy. The government said the increased spending will take Australia's defense budget to 3 percent of GDP by 2033, under the NATO methodology, which includes related spending such as military pensions.

Marles detailed the new strategy in a speech to the National Press Club in Canberra, saying the government was undertaking the most ambitious modernization of Australia's maritime capability since World War II.

Chen Hong, director of the Australian Studies Centre at East China Normal University in Shanghai, said the so-called partnership between Tokyo and Canberra is gradually shedding its neutral character and evolving toward substantive joint operational capabilities.

"Japan-Australia ties have expanded from security dialogues and the Reciprocal Access Agreement to joint exercises, intelligence sharing, and defense industrial cooperation, signaling the rapid development of an institutionalized security network," he said.

"The procurement of Mogami-class frigates represents more than a conventional arms transaction; it marks a critical step toward deeper integration of the two countries' military systems," he added.

In effect, Japan-Australia ties are evolving into a de facto "quasi-alliance" — unofficial but real — allowing both sides to avoid the costs of a formal pact while still moving closer to one, Chen said.

He said Australia's deepening security ties with Japan provide it with a new military foothold and reinforce its policy narratives on supply chain and energy security, "but at the cost of a gradual erosion of its strategic autonomy".

Japan's closer military ties with Australia signal efforts to align with the US-led frameworks like AUKUS — a trilateral partnership between Australia, the UK and the US — aiming to boost capabilities and expand its regional military role through technology cooperation and intelligence sharing, said Liu Shuliang, an associate researcher at the Tianjin Academy of Social Sciences.

Japan and Australia, both key US allies, have actively aligned with US' strategic priorities, using such "small blocs" to deepen military cooperation, Liu said.

More seriously, the Japan-Australia security alignment is spilling over into the broader Asia-Pacific security architecture, experts said.

"Grounded in Cold War mentality, such a framework weakens the central role of multilateralism in safeguarding regional security, constrains the scope for multilateral cooperation, and diminishes the effectiveness of collective security mechanisms," Liu said.

It also risks aggravating regional flashpoints, accelerating destabilizing trends, fueling arms races, and deepening the regional security dilemma, he added.

The introduction of an exclusive "quasi-alliance" framework would likely intensify tensions and deepen the security dilemmas confronting Japan and Australia, said Chen.

Agencies and Ji Haisheng in Beijing contributed to this story.