The defence minister, Richard Marles, has said the “seabed is a battlefield” in a combative speech urging Beijing to be more transparent about its maritime operations, and taking aim at weak international controls over so-called “shadow-fleet” vessels.

The warning came as the US, UK and Australia announced a new Aukus project to develop new underwater drone technology to protect undersea cables.

The same announcement also revealed that Australia would buy three secondhand Virginia-class submarines from the US under Aukus, instead of a mix of old and new, in a move to “simplify supply chain management, operational and maintenance requirements, and maximise cost efficiencies”.

Speaking at the Shangri-La Dialogue in Singapore, Marles said undersea internet cables – “the arteries of modern civilisation” – were being cut at an unprecedented rate, with island nations like Australia acutely vulnerable.

“The seabed is becoming a battlefield. The shadow fleet is becoming a weapon,” he told Asia’s largest defence summit.

“Over the past 18 months, we have witnessed a series of attacks against subsea critical infrastructure at a scale and frequency that is historically unprecedented.

“This is not speculation. This is a documented pattern of behaviour. And we must reckon with it honestly.”

Marles cited five cases of cables being cut in the Taiwan Strait in the past 18 months, attributed to China, and three in the Baltic Sea, alleged to have been committed by Russia.

“Now, maybe these were accidents. But even if they were, it highlights the vulnerability of this crucial part of the globe’s infrastructure,” Marles said.

“If they were intentional, we are left to wonder: are countries testing our response times, testing our attribution thresholds and testing our political will to respond?”

The US secretary of defence, Pete Hegseth, announced the first “signature project” for the second pillar of the Aukus treaty, whose first pillar is the nuclear submarine project.

“This signature project will deliver a suite of highly adaptable multi-mission UUV payloads designed to support undersea operations and maintain our collective advantage in the maritime domain,” he told reporters at a briefing in Singapore.

The UK defence secretary, John Healey, said that the planned technology, a “range of cutting edge sensors and weapons systems” for underseas drones, “will rapidly give our forces the very most advanced battlefield technologies”.

The systems will be deployed on uncrewed underwater vessels, Healey added.

About 99% of Australia’s internet traffic flows through just 15 subsea cables, Marles said.

“Our financial systems, our health systems, our communications, our intelligence partnerships, our ability to operate as a modern economy and a functioning state: all of it is critically dependent on infrastructure that is exposed, that cannot move and … can be cut with an anchor in the middle of the night.”

Marles said China had “a real opportunity” to contribute to a more stable Asia-Pacific region.

“A commitment to transparency around its maritime operations would be a meaningful contribution to the regional stability upon which China’s own prosperity depends.

“Existing patterns of grey zone activity are not consistent with a peaceful and stable regional order.”

Marles said the problem of “shadow-fleet” ships – vessels operating in the grey zone between commercial shipping and instruments of state coercion – extended far beyond the vulnerability of subsea cables.

“These same networks of unregistered, flag-of-convenience vessels are vectors for sanctions evasion, for the transport of energy that sustains Russia’s war in Europe, for illegal fishing, for human and drug trafficking.”

Hegseth said the Trump administration would not let China dominate the Pacific, but did not directly mention Taiwan.

Hegseth poured further pressure on allies in Europe and Asia to spend more on defence, arguing the US wanted “partners not protectorates”. And he invoked Theodore Roosevelt by declaring America would “speak softly, but carry a big stick”.

Hegseth said the US wanted to work with allies to create a “stable equilibrium” in the Asia-Pacific.

“A Pacific dominated by any hegemon would unravel the regional balance of power and undermine the equilibrium we all seek to preserve,” Hegseth said.

“The Department of War [the unofficial alternate name for the US department of defence] is working with the utmost focus to prevent any such unravelling.”

He warned the Trump administration’s record defence budget request – US$1.5tn (A$2.085tn) – would “unleash America’s arsenal of freedom and expand America’s military dominance for decades to come”.

But he did not directly mention Taiwan, the issue most keenly watched by officials and ministers from other countries, in the wake of an underwhelming summit between Donald Trump and Xi Jinping in Beijing this month, as well as America’s stalled arms sales to Taiwan.

There is concern in Taiwan that the Trump White House’s commitment to the autonomous island is less robust than that of previous US administrations.

Last year, Hegseth was full-throated in his position, warning against the “real” and potentially “imminent” threat of a Chinese invasion of the island. He said China’s army was “rehearsing for the real deal”.

Beijing’s “One China” principle regards Taiwan as an inalienable part of its own territory and has vowed “reunification” with the democratic and autonomously governed island, by force if necessary.

Taiwan’s government rejects Beijing’s sovereignty claims, insisting the island’s people must determine their future.