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.