U.S. warships have sailed into disputed waters in the South China Sea, according to military analysts, heightening a standoff in the waterway and sharpening the rivalry between the United States and China, even as much of the world is in lockdown because of the coronavirus.

The USS America, an amphibious assault ship, and the USS Bunker Hill, a guided missile cruiser, entered contested waters off Malaysia. At the same time, a Chinese government ship in the area has for days been tailing a Malaysian state oil company ship carrying out exploratory drilling. Chinese and Australian warships have also powered into nearby waters, according to the defense experts.

Despite working to control a pandemic that spread from China earlier this year, Beijing has not reduced its activities in the South China Sea, a strategic waterway through which one-third of global shipping flows. Instead, the Chinese government’s yearslong pattern of assertiveness has only intensified, military analysts said.

“It’s a quite deliberate Chinese strategy to try to maximize what they perceive as being a moment of distraction and the reduced capability of the United States to pressure neighbors,” said Peter Jennings, a former Australian defense official who is the executive director of the Australian Strategic Policy Institute.

Since January, when the coronavirus epidemic began to surge, the Chinese government and coast guard ships, along with maritime militias, have been plying contested waters in the South China Sea, tangling with regional maritime enforcement agencies and harassing fishermen.

Earlier this month, the Vietnamese accused a Chinese patrol ship of ramming and sinking a Vietnamese fishing boat.

After the sinking of the Vietnamese boat, the State Department urged China in a statement “to remain focused on supporting international efforts to combat the global pandemic, and to stop exploiting the distraction or vulnerability of other states to expand its unlawful claims in the South China Sea.”

The Chinese government has made vast claims to the South China Sea that conflict with demarcations made by five other governments. An international tribunal has dismissed most of China’s claims to the waterway, but Beijing does not recognize the ruling and has instead built naval bases on reefs it now controls.

While the United States has no territorial claims in the South China Sea, the U.S. Navy says it has kept the peace in these waters for decades. U.S. military officials have chastised China for its increased militarization of the waterway.

“Through our continued operational presence in the South China Sea, we are working with our allies and partners to promote freedom of navigation and overflight, and the international principles that underpin security and prosperity for the Indo-Pacific,” said Lt. Cmdr. Nicole Schwegman, a spokeswoman for the U.S. Indo-Pacific Command. “The U.S. supports the efforts of our allies and partners to determine their own economic interests.”

The Chinese government has countered that the United States is the country destabilizing the region. The appearance of the America and the Bunker Hill may do little to dispel that narrative.

And regional governments have worried that the United States has a habit of briefly showing up in hot spots only to depart, leaving them to contend with an increasingly muscular Beijing.

The area where the American warships have been sailing is around 200 nautical miles off the coast of Malaysia, defense experts said. Malaysia, China and Vietnam all claim rights to the natural resources in this part of the contested waterway.

Last week, a Chinese government survey ship began shadowing the West Capella, a drill ship conducting exploration activities off the Malaysian coast and operated by Petronas, the Malaysian state oil company.

The Chinese survey ship, called the Haiyang Dizhi 8, had previously tracked similar oil operations off Vietnam.

An Australian frigate, the HMAS Parramatta, is accompanying the American naval ships, as part of a previously planned operation, according to defense experts.

Jennings, the former Australian defense official, said that the Parramatta’s deployment would have been arranged at least a year ago.

At that time, “it probably didn’t know it was sailing into a heightened military environment,” Jennings said. “It’s been made that way really since March, with the greater pattern of offensive operations that China is engaging in all the way from Japan to the South China Sea.”

Defense experts who have reviewed information about military movements in the area but are not authorized to share them publicly, said that a Chinese warship has been operating off the coast of Malaysia. The destroyer is called the Wuhan, named after the city where the coronavirus outbreak began.