A Chinese research vessel flanked by coast guard and nearly a dozen boats on Wednesday entered a gas block operated by Russian and Vietnamese state firms, two monitoring groups said, another potential flashpoint in the South China Sea.
It follows a pattern of assertive moves of late by Beijing in its neighbors' exclusive economic zones (EEZ), as it presses its claim to sovereignty over almost the entire South China Sea, testing the United States and its allies at a time of heightened regional tension.
Strong relations between Russia and China are a major factor supporting global stability, Russian Defence Minister Sergei Shoigu said in March, but the two countries have conflicting interests in the South China Sea.
The Chinese research ship, two coast guard and 11 fishing boats entered block 04-03 of Vietsovpetro, a venture between Russia's Zarubezhneft and PetroVietnam, and were still in the area by nightfall, data from two independent vessel monitors reviewed by Reuters showed.
The Chinese group was also close to blocks 05-1 B and 05-1 C, operated by Idemitsu Oil & Gas, a unit of Japan's Idemitsu Kosan, the data showed.
China has been accused of using its coast guard and an armada of fishing boats widely considered militia to intimidate and disrupt energy activities, including off Malaysia, Indonesia, the Philippines and Vietnam. Beijing says it is lawfully operating in its waters.
But the move on Wednesday was "extraordinary", according to Ray Powell, who leads Stanford University's Project Myoushu on the South China Sea, because of "the sheer number of militia and coast guard ships involved".
"It seems they are sending a message about China's jurisdiction over Vietnam's oil and gas operations," Powell said, adding at least three Vietnamese fisheries surveillance vessels had moved close to the Chinese ships.
'China's jurisdiction'
China's foreign ministry said those activities were "normal".
"Chinese fishing and scientific research vessels carry out normal production and work activities in maritime areas under China's jurisdiction," it said.
It made similar remarks on Tuesday after Chinese boats approached an area where the navies of India and Southeast Asian countries held exercises.
Japan's Idemitsu declined to comment. Vietnam's foreign ministry and the other companies involved did not immediately respond to requests for comment.
On Wednesday, the Chinese vessels were about 10 nautical miles (18 km) from the Japanese operating rig and about 20 miles from the Russia-Vietnam rig, according to South China Sea Chronicle Initiative (SCSCI), an independent non-profit.
The Chinese research vessel moved at full speed before entering Vietnam's EEZ but slowed to 4-5 knots, suggesting the ship was conducting a survey there, said SCSCI's Van Pham.
Surveys in a country's EEZ's without prior notification have in previous instances been considered hostile or provocative.
It follows similar incidents in March in two blocks involving Russian firms off Vietnam, which has licensed operations of more than 150 blocks
(Reuters - Reporting by Francesco Guarascio; additional reporting by Laurie Chen and Khanh Vu; Editing by Martin Petty)