The US has accused China of increased "provocations" in the South China Sea, saying it is only a matter of time before the country's "aggressive and irresponsible behaviour" leads to a major incident or accident.
The countries have had territorial disputes for decades but tension has risen in recent years.
Jung Pak, deputy assistant secretary for East Asia at the State Department, said there was a "clear and upward trend" of Chinese provocations against other countries claiming parts of sea, as well as other states operating legally in the region.
There have been three separate incidents in the last few months where China challenged marine research and energy exploration within the exclusive economic zone claimed by the Philippines in the sea, she said at an event by the Center for Strategic and International Studies think tank.
Ely Ratner, assistant secretary of defence for Indo-Pacific Security Affairs, said there have been "dozens" of incidents involving the Chinese military in the South China Sea during the first half of this year - a sharp increase over the past five years.
"Beijing is systematically testing the limits of our collective resolve," he told the same event.
"In my view, this aggressive and irresponsible behaviour represents one of the most significant threats to peace and stability in the region today, including in the South China Sea."
"And if the PLA continues this pattern of behaviour, it is only a matter of time before there is a major incident or accident in the region," he said, referring to China's armed forces.
'Expansive and unlawful claims'
The growing US-China strategic rivalry is expected to be a focus of a call between US President Joe Biden and Chinese President Xi Jinping this week.
The leaders are expected to discuss how to prevent conflict, particularly over the island of Taiwan.
China has warned it could take control of the democratically-ruled territory through force if necessary.
The US will join foreign ministers and partners from Southeast Asia when they meet in Cambodia later in the week.
Ms Pak said China's "provocative actions" to implement its "expansive and unlawful" claims over the South China Sea "contribute to regional instability, damage the economies of other claimant states, undermine the existing maritime order, and threaten the rights and interests of all nations that rely on or operate in this vital waterway".
She said Washington had a "very complicated relationship with Beijing", which is why the US was not trying to counter all of China's actions in Southeast Asia and the rest of the developing world.
"We want to make sure that countries, as they have their relationships with Beijing, have the tools and the power, and the ability, to stand up for their autonomy and their sovereign decision making," she said.