Vietnam's coast guard has seized a Chinese fuel ship for allegedly intruding into its waters, report state media.
The ship, carrying more than 100,000 litres of fuel, was stopped near the northern maritime border last week.
Vietnamese media say the crew admitted they were supplying fuel to Chinese fishing boats in the area.
Vietnam and China are locked in a dispute over overlapping claims in the South China Sea.
The government-owned Vietnam News Agency said the seized vessel had been disguised as a fishing boat.
It was discovered in waters near the Vietnamese island of Bach Long Vi and the Chinese island of Hainan last Thursday.
The captain and two crew members have since been arrested, said the reports.
Vietnamese newspapers quoted authorities as saying the crew did not produce the required work permits and documentation on the origin of the oil.
There has been no comment from China.
Hanoi has frequently complained about the presence of Chinese fishing vessels in the disputed areas, as well as the harassment of its own fishing boats by Chinese authorities.
China has angered several of its neighbours with growing assertiveness over its claims in the South China Sea, with aggressive land reclamation on disputed reefs and islands and the increasing presence of civilian vessels and coast guard boats.