These reach to within 37 nautical miles (68km) of Vietnam's coast, according to PetroVietnam, a Vietnamese state-owned oil firm.

ECONOMIST: The South China Sea

State-owned PetroVietnam plans to keep buying foreign oil and gas assets.

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In late May, the Vietnamese Ministry of Defense reported that a Chinese patrol boat slashed a submerged cable of a oil and gas survey ship operated by PetroVietnam, the state energy firm.

CNN: Explainer: South China Sea - Asia's most dangerous waters