Bejing has placed two of its coast guard ships near ONGC's block.
In a third major intrusion, China placed twenty ships in Vietnam's exclusive economic zone.
Critically, the area has India's commercial interest as ONGC Videsh Ltd has its oil block with shareholding from Russia's Rosneft in the economic zone.
Bejing has placed two of its coast guard ships near ONGC's block. This is the second intrusion by the Chinese this year which began on August 13 at 1500 hours(local time).
In the second intrusion, six coast guard ships, ten fishing ships and two service ships along with H6 bomber, fighter aircraft and mid-air refuelers were also spotted.
The Chinese have also placed speakers asking Vietnam drillers to leave.
"Escalation is building up..blunt violation of International govt", a Vietnamese government source said.
The first intrusion began on July 3 and Chinese ships left on August 7. The number of ships at that time were thirty-five, but without much airpower. Interestingly, the Chinese deployment was removed after the ASEAN foreign ministers meet in ASEAN on August 1.
The meet saw the participation of over twenty foreign ministers of the region including the Indian external affairs minister S Jaishankar and China's Wang Yi.
"Very well-calculated action and at the Bangkok meeting, Chinese side said there is peace and security in the region..deep down this is the destruction of freedom of navigation", the source added.
After the first intrusion, Chinese envoys were contacted thirty times, and since August 13 intrusion, seven times the Chinese envoys have been contacted by the Vietnamese side.
Hanoi is mulling a number of actions including legal and going to United Nations Security Council.
"Just follow the same China Pakistan model at UNSC.. if Pakistan can take a bilateral issue at UNSC, South China Sea is still an international issue", a diplomat said.
Indian and Russian governments who have a commercial interest in the area have been informed. Vietnamese envoys have reached out to global capitals and the country is in touch with its partners like US, Australia and its 16 dialogue partners.
This is the third major intrusion by the Chinese since 2011. In 2011, Chinese had cut Vietnamese cables and in 2014, Beijing had placed a drilling platform, which was later withdrawn.