The United States on Tuesday warned China that a decision to move a deep-sea oil rig into disputed waters in the East Vietnam Sea was a "provocative" step which it was monitoring closely.

"We're looking carefully into this matter," State Department deputy spokeswoman Jen Psaki told reporters.

Given the recent history of tensions in the East Vietnam Sea, China's decision to operate its oil rig in disputed waters is “provocative and unhelpful to the maintenance of peace and stability in the region,” Psaki said.

Vietnam has labeled China's decision "illegal" and demanded the rig be withdrawn.

Psaki said the renewed regional tensions highlighted the "need for claimants to clarify their claims in accordance with international law and reach agreement about what types of activities should be permissible within disputed areas."