China National Offshore Oil Corp. towed the rig, known as HYSY 981, into a disputed area of the South China Sea in May 2014, triggering an uproar in Vietnam. Anti-Chinese riots broke out in several parts of the country while Chinese and Vietnamese coast-guard and fishing vessels stood off against each other at sea.

In a statement on Vietnam’s government website, foreign ministry spokesman Le Hai Binh said China last pulled the rig to its present location in the mouth of the Gulf of Tonkin on Sunday. It was towed to a nearby location in January.

“Vietnam strongly protests this and demands that China drop its drilling plans and move it out of the area,” Mr. Binh said. He said Vietnam had lodged an official protest with China’s embassy in Hanoi.

In Beijing, Foreign Ministry spokesman Hong Lei told a news briefing on Friday that the “operation is in undisputed Chinese waters and it is a normal, commercial exploration. We hope relevant countries would take an objective and reasonable view on this.”

This latest spat comes as relations between the two countries are deteriorating again. Vietnamese authorities last week seized a Chinese ship they said had illegally entered Vietnamese waters carrying 100,000 liters of diesel fuel. State media in Hanoi reported the captain of the three-man vessel as saying the boat was supplying oil to Chinese fishing boats in the area.

It also comes as Vietnam completes its new leadership lineup.

The country’s legislature on Thursday formally elected Nguyen Xuan Phuc as prime minister, replacing Nguyen Tan Dung, an outspoken and popular critic of China’s military and commercial expansion in the South China Sea in recent years.

Mr. Phuc joins State President Tran Dai Quang and Communist Party Secretary-General Nguyen Phu Trong, who has grown more critical of China’s role in the region, as the third of the country’s top leaders.

By Vu Trong Khanh - The Wall Street Journal - April 8, 2016

China rebuffs Vietnam criticism of oil rig move

China's Foreign Ministry on Friday rebuffed Vietnam's second demand this year to move a controversial oil rig and drop plans to drill in South China Sea waters where jurisdiction is unclear, saying it was engaging in normal exploration activity.

The $1-billion rig, which was at the center of a fierce diplomatic stand-off between the countries in 2014, had moved into an area of the Gulf of Tonkin over which, Vietnam said, the two countries were still "executing delineation discussions".

"The relevant work is in undisputed Chinese waters, and it is normal commercial exploration," Chinese Foreign Ministry spokesman Hong Lei told a daily news briefing. "We hope the relevant party takes an objective and reasonable view on this."

He did not elaborate.

China claims most of the resource-rich South China Sea amid rival claims by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

Two years ago, China parked the rig, the Haiyang Shiyou 981, for 10 weeks in waters Vietnam considers its exclusive economic zone, triggering their worst row in decades and an outcry among Vietnamese nationalists.

Many experts call the move a miscalculation by Beijing that played into the hands of the United States. Since the row, Vietnam has become closer to Washington than ever before.

Vietnam closely tracks the movement of the oil rig, which has operated as far away as the Bay of Bengal, and has been close to disputed waters several times since 2014.

Both of Vietnam's protests this year against the rig's activity have coincided with leadership changes in Hanoi.

Vietnam swore in a new prime minister on Thursday and a new president last week. Its previous complaint about the rig was in January, two days before the start of its Communist Party's five-yearly congress.

Vietnam has also criticized China's decision to start operating a lighthouse on one of its artificial islands in the Spratly archipelago, saying it violated Vietnam's sovereignty and was illegal.

Hong said the lighthouse was a matter for China, but it had been built to improve navigational safety for all users of the South China Sea.

By Jessica Macy Yu - Reuters - April 8, 2016