Some oil blocks off Vietnam's sprawling coastline fall within an area of the South China Sea demarcated by China's "nine-dash line," the basis for Beijing's controversial claims to most of the resource-rich waterway.

Last week, sources said Rosneft Vietnam BV, a unit of Russian state oil firm Rosneft, was concerned its recent drilling in one such block could upset Beijing.

That prompted Vietnam's foreign ministry to assert the blocks are "entirely under Vietnamese sovereignty and jurisdiction," and a warning from Beijing to respect its sovereign rights.