Vietnam has begun drilling for oil in an area of the resource-rich South China Sea also claimed by China.
An oil industry consultant told the BBC that a drilling ship on contract to international firm Talisman-Vietnam was working off Vietnam’s south-east coast.
This appears to be why a senior Chinese general cut short an official visit to Vietnam last month.
China claims almost all of the South China Sea, including reefs and islands also contested by other nations.
According to Ian Cross, of Singapore-based Moyes & Co, the drillship Deepsea Metro I, began to drill in an area of sea about 400km (250 miles) off the Vietnamese coast on 21 June.
It is likely that the news was kept secret because of its extreme sensitivity.