American 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 America, an amphibious assault ship, and the 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.