HANOI, Vietnam – Vietnam warned China on Tuesday that it would take all necessary measures to defend its interests in the South China Sea if Beijing does not remove a large oil rig from waters claimed by both countries.

China’s stationing of the oil rig over the weekend is widely seen as one of its most provocative steps in a gradual campaign of asserting its sovereignty in the South China Sea.

China’s assertiveness along with its growing military and economic might is alarming Vietnam, the Philippines and other countries in the region. The United States, which is undertaking a military and economic “pivot” toward Asia in part to counter Chinese influence, shares the concerns.

In Washington, State Department spokeswoman Jen Psaki called China’s action “provocative and unhelpful to the maintenance of peace and stability in the region.”

A Vietnamese government statement said Foreign Minister Pham Binh Minh called Chinese State Councilor Yang Jiechi and told him the deployment of the $1 billion deep-sea rig was illegal and a violation of Vietnamese sovereignty.

Beijing says that the rig, CNOOC 981, is in its territorial waters. China claims nearly all of the South China Sea.