Vietnam started dredging work on the disputed Ladd Reef in the South China Sea, setting up a tense face-off with China, which claims the entire waters.
Satellite images taken on November 30 showed several vessels at work in a newly dug channel between the lagoon and open sea at Ladd Reef, which is on the southwestern fringe of the Spratly group of islands, Japan Times reported.
Observers said the current dredging work is most likely in preparation for extensive construction on the reefs. Vietnam has already built a lighthouse and an accommodation facility for soldiers on the reef, which is claimed both by China and Taiwan.
China scored decisive military victories over Vietnam in recent decades in its quest to cement claims over the island chains. In 1974, more than 70 Vietnamese soldiers died in China's military offensive that seized control of Paracels. In 1988 Vietnam lost Spratlys to China in another bloody military conflict.
In 2014, China set up a drilling rig in waters near the Paracel Islands, angering Vietnam. More than 20 people were killed in protests in Vietnam after China's rig move. China withdrew the rig after conducting drilling for two months. In 2015 satellite images showed Beijing was building an airstrip on land reclaimed around the Spratlys.
In January Vietnam objected to China moving its Haiyang Shiyou oil rig to an area of the waters where the two countries' continental shelves overlap. The rig was at the centre of a fierce diplomatic row between the communist neighbours that ad also resulted in violent anti-China protests in Vietnam that claimed the lives of more than 20 people.
At the core of the South China Sea conflict is the overlapping claims of sovereignty over two island chains -- Paracels and Spratlys -- and waters around them. China, Taiwan, Malaysia, the Philippines, Vietnam and Brunei have made claims to a clutch of shoals, rocky outcrops, atolls and sandbanks in the sprawling South China sea.