China's mounting opposition comes less than a week before planning commission deputy chairman Montek Singh Ahluwalia will lead a high-level team to Beijing from September 24 to 26 for the first India-China Strategic Economic Dialogue.
HT had broken the story last week about the diplomatic row between India and China over OVL's activities in what New Delhi described as Vietnamese blocks in the South China Sea.
Beijing issued a demarche to New Delhi over the issue and a Chinese foreign ministry spokesperson expressed her country's objections to OVL's exploration a day later.
Hong said at a daily media briefing on Monday, "Any country engaged in oil and gas exploration activities in this jurisdiction without the approval of the Chinese government constitutes an infringement upon China's sovereignty and national interest and are therefore illegal and invalid."
India's stand has been that the exploration by OVL is being carried out in two Vietnamese blocks in accordance with international rules and would continue.
External affairs minister SM Krishna had during his visit to Hanoi last Friday said that the energy sector, including oil and gas exploration, would be the key area of cooperation between India and Vietnam.