U.S. Secretary of State Mike Pompeo called the decision a “strengthening of U.S. policy,” saying in a statement that “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.”

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

The Chinese embassy in the U.S. said the move is “completely unjustified,” adding that the U.S. is “flexing muscles, stirring up tension and inciting confrontation in the region” under the pretext of preserving stability.

The U.S. and China have faced off over the coronavirus pandemic and China’s actions in Hong Kong and Xinjiang in recent weeks, creating an increasingly strained relationship between Washington and Beijing.

China claims almost all of the 1.3 million square mile South China Sea under its self-proclaimed “nine-dash line” and has built up military fortifications on several islands in the sea over the past few years.

The Trump administration has been increasing naval operations in the area to challenge the claims, this month sending two aircraft carriers to take part in one of its largest naval exercises in the area in recent years at the same time that China was holding drills.

The Philippines, Vietnam, Brunei, Malaysia and Taiwan also claim all or part of the sea, which sees $5 trillion in goods ship through each year. The United States had previously urged a peaceful resolution to the dispute through UN-backed arbitration.

Pompeo’s announcement came a day after the fourth anniversary of an international arbitration tribunal’s ruling that found there was no legal basis for China’s claims to historic and economic rights in most of the South China Sea. Last month Southeast Asian leaders released a joint statement insisting that the 1982 United Nations Convention on the Law of the Sea should be the basis for determining sovereign rights and entitlements in maritime areas, the Wall Street Journal reported.

“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.”