India is being pulled into a complex and increasingly tense territorial dispute in the South China Sea, with China repeatedly warning ONGC, the Indian state oil company, that its joint exploration plans with Vietnam amount to a violation of Chinese sovereignty.

The Indian government responded to the latest Chinese warnings Thursday by repeating its pledge to continue exploring for energy in the South China Sea, where China is embroiled in territorial disputes with Vietnam, the Philippines, Taiwan, Malaysia and Brunei.

ONGC,...