State-run China National Offshore Oil Corp. last week positioned an oil rig about 120 miles off the coast of Vietnam in waters claimed by both China and Vietnam. China says the rig is operating in territory under its control, while Vietnam says it reserves the right to take action to protect its sovereignty.
U.S. State Department spokeswoman Jen Psaki said Wednesday the Chinese move was provocative.
"This unilateral action appears to be part of a broader pattern of Chinese behavior to advance its claims over disputed territory in a manner that undermines peace and stability in the region," she said in a statement.
State-owned PetroVietnam sent a letter to CNOOC last weekend "strongly opposing" the deployment of drilling rig HD-981, saying it violated the spirit of bilateral relations.
"PetroVietnam requested CNOOC not to recur similar actions," it said.
Vietnamese and Chinese vessels collided Wednesday, the third time in several days, as Vietnam attempted to prevent China from drilling in contested waters. Vietnamese officials claimed the Chinese ships intentionally rammed their vessels.
China is at odds with its regional neighbors over claims to territory in the South China Sea.