US Secretary of State Mike Pompeo, seen here in November 2018, has sharply criticised China over the South China Sea. — AFP/VNA Photo

WASHINGTON — Secretary of State Mike Pompeo said on Monday that the United States would treat Beijing's pursuit of resources in the dispute-rife South China Sea as illegal, ramping up support for Southeast Asian nations.

It was the latest forceful statement by President Donald Trump's administration to challenge China.

"We are making clear: Beijing's claims to offshore resources across most of the South China Sea are completely unlawful, as is its campaign of bullying to control them," Pompeo said in a statement.

"The world will not allow Beijing to treat the South China Sea as its maritime empire."

The United States has long rejected Beijing's sweeping claims in the South China Sea, which is both home to valuable oil and gas deposits and a vital waterway for the world's commerce.

Pompeo's statement goes further by explicitly siding with Southeast Asian nations including the Philippines and Việt Nam, after years of the United States saying it took no position on individual claims.

"America stands with our Southeast Asian allies and partners in protecting their sovereign rights to offshore resources, consistent with their rights and obligations under international law," Pompeo said.

"We stand with the international community in defence of freedom of the seas and respect for sovereignty and reject any push to impose 'might makes right' in the South China Sea or the wider region."

China earlier this month defended itself against US criticism over Beijing's military exercises in the South China Sea, saying its activities were "within the scope of China's territorial sovereignty."

Beijing hit back Tuesday at the US criticism, saying the accusation was "unjustified' and a bid to sabotage regional peace.

"We advise the US side to earnestly honour its commitment of not taking sides on the issue of territorial sovereignty, respect regional countries' efforts for a peaceful and stable South China Sea and stop its attempts to disrupt and sabotage regional peace and stability," the Chinese embassy in Washington said in a statement.

Beijing claims the majority of the South China Sea through the so-called nine-dash line, a vague delineation based on maps from the 1940s when the Republic of China snapped up islands from Japanese control.

Pompeo issued his statement to mark the fourth anniversary of a tribunal decision that sided with the Philippines against the nine-dash line.

Pompeo said that China, based on the court decision, cannot make claims based on the Scarborough Reef or Spratly Islands (known as Trường Sa in Việt Nam), a vast uninhabited archipelago.

The United States as a result now rejects Beijing's claims in the waters surrounding Vanguard Bank off Việt Nam, Lucania Shoals off Malaysia, waters considered in Brunei's exclusive economic zone and Natuna Besar off Indonesia, Pompeo said.

"Any PRC action to harass other states' fishing or hydrocarbon development in these waters – or to carry out such activities unilaterally – is unlawful," Pompeo said.

Pompeo also rejected Beijing's southernmost claim of James Shoal, some 1,800km (1,150 miles) from the Chinese mainland, saying the speck administered by Malaysia was completely submerged by water and therefore cannot determine a maritime zone.

The 2016 decision was issued by a tribunal under the UN Convention on the Law of the Sea. Pompeo noted that China is a party to it and called the ruling legally binding.

The United States, however, is one of the few countries that is not part of the convention, with conservatives opposing any loss of autonomy to a global body. AFP