Chinese diplomats in Washington have warned oil giant ExxonMobil against pursuing an oil-exploration deal off the coast of Vietnam in waters China considers its own, the South China Morning Post reported, citing sources close to the company. The diplomats have made several verbal protests to ExxonMobil executives, reportedly saying that future business on the mainland could be at risk. ExxonMobil and state-owned energy firm PetroVietnam have signed a preliminary agreement to cooperate in exploration projects in the South China Sea off Vietnam’s south and central coasts, including some areas to which China says it has a historical claim. China has also protested a similar project conducted by BP and Vietnam. That project stalled, but is preparing to resume, said BP officials.