Tensions between China and the United States have risen further in recent days over the Taiwan Strait.
Chinese officials have been increasingly asserting that the Taiwan Strait is not international waters, while US officials have reportedly decided to reject that assertion, concerned that Beijing's stance could lead to more contestation in the region.
China is already locked in territorial disputes with Brunei, Malaysia, the Philippines, Vietnam and Taiwan over the South China Sea, which is home to key commercial shipping lanes and valuable oil and gas reserves, as well as rich fishing grounds.To protect its economic and security interests, the US has challenged China's claim and has accused it of bullying.
The Taiwan Strait is a waterway separating China from the main island of Taiwan, measuring 70 nautical miles at its narrowest point and 220 nautical miles at its widest.
At its widest, it is approximately the length of Taiwan from its northernmost to southernmost tips.
China regards Taiwan as a renegade province awaiting reunification with the mainland, by force if necessary, and the strait is of strategic importance as it is a vital sea line for trade and other maritime activities conducted by vessels from many key ports around the region.
What's the US stance?
The US and its allies assert that much of the Taiwan Strait constitutes international waters, and they have routinely sent their warships through the waterway as part of freedom of navigation exercises, angering China.
"The Taiwan Strait is an international waterway" where freedom of navigation and overflight "are guaranteed under international law", US State Department spokesman Ned Price told the media on June 19.
In a Bloomberg report on June 12 highlighting Washington's concern over Chinese officials' private warnings to their US counterparts to avoid sailing through the waterway, Pentagon spokesman Martin Meiners said: "The United States will continue to fly, sail and operate wherever international law allows, and that includes transiting through the Taiwan Strait."
What about China's stance?
"China enjoys sovereign rights and jurisdiction over the Taiwan Strait, while respecting the legitimate rights of other countries in the relevant maritime areas," China's Foreign Ministry spokesman Wang Wenbin said on June 13.
"There is no such thing as 'international waters' in Unclos. By claiming that the Taiwan Strait is international waters, some countries intend to create an excuse for its manipulation of the Taiwan issue and threaten China's sovereignty and security," he added.
Unclos refers to the United Nations Convention on the Law of the Sea, an international agreement that sets out a universal legal framework on the governance of the ocean and maritime activities.
Most UN member states - 164 out of 193 - have ratified Unclos, including China; but the US remains among the holdouts, although it recognises the convention as customary international law.
Taiwan's Foreign Ministry spokesman Joanna Ou has called Beijing's stance a "distortion of international law", and said it "revealed China's ambitions to annex Taiwan".
What exactly are 'international waters'?
Informally, "international waters" refer to waters beyond any state's territorial sea.
But it is not actually a defined term in Unclos, as China rightly points out.
According to Unclos, all states have the right to establish their territorial sea up to 12 nautical miles from the baseline of their shores. Foreign ships, meanwhile, have the right of innocent passage through any country's territorial sea.
Beyond the territorial sea is a contiguous zone - up to 24 nautical miles from the coastline - within which a state can exercise controls to prevent actions that may infringe on its Customs, fiscal, immigration or sanitary laws as well as regulations within its territory.
Unclos also defines an exclusive economic zone (EEZ) - up to 200 nautical miles from the shore - within which a state has the sovereign rights to exploration, use of natural resources, energy production and economic activities. Within any country's EEZ, however, foreign states have freedom of navigation and overflight, as well as to lay submarine cables and pipelines.
Beyond the EEZs are what Unclos calls the high seas, which fall outside any national jurisdiction.
The US is not wrong in saying that, regardless of whether the Taiwan Strait is considered "international waters" or an EEZ, foreign ships have the right to transit through the waterway as international law allows them to do so.
Why the spat now, and why does it matter?
While Beijing regularly protests against Washington's routine freedom of navigation operations in the Taiwan Strait, the legal status of the waterway was previously not a focal point in bilateral meetings between the two powers.
News of China's move to reject the idea of the Taiwan Strait being "international waters" came the same week it reminded the US at Asia's top security summit, the Shangri-La Dialogue, that it would "not hesitate to fight to the very end" over Taiwan if provoked.
The timing of Beijing's increased assertion is causing concern in Washington, given the already-tense global security environment after Russia's invasion of Ukraine on Feb 24.
Bloomberg has reported that some US officials believe China is gauging the response in Washington to the Ukraine crisis as a proxy for how the US would deal with more aggressive action by Beijing against Taiwan.
Academic Collin Koh, a research fellow specialising in naval affairs and maritime security at Nanyang Technological University in Singapore, said there should not be any dispute over the Taiwan Strait as "all along… the free movement of ships through the strait has generally gone unimpeded".
He said "interrelated problems" lie at the root of the current dispute.
These include "broader, underpinning Sino-US tensions" dating to the Trump administration (2017 to 2021), "when the US government started to openly assert freedom of navigation in the strait even though it has been established practice, even for military vessels", as well as "a stronger People's Liberation Army… that behoves Beijing to respond more resolutely".
Whatever the root causes, greater tensions between the two powers in the Taiwan Strait raise the risks of a confrontation, contributing to overall instability in the region.