Vietnam has started dredging work on a reef in the South China Sea, fresh satellite images appear to show, a move that could provoke Beijing which claims most of the disputed waterway.

An image of Ladd Reef in the Spratly Islands, where Vietnam has a lighthouse, shows several vessels in a carved out embankment.

Sediment can be seen leaking out into the ocean, according to the November 30 image provided to AFP on Friday by US-based Planet Labs, a satellite imaging company.

A July picture from Planet Labs shows no breach of the reef's embankment, suggesting the work began in recent months on the small piece of land that is also claimed by Taiwan.

Vietnamese officials did not respond to a request for comment.

The images follow photos published last month from US-based Asia Maritime Transparency Initiative that showed Vietnam had extended a runway and was building hangars capable of hosting military equipment on a different island in the Spratlys also claimed by Beijing.

But Vietnam's latest moves remain small-scale compared to China's build-up in the waterway, where it has constructed several islands capable of supporting military facilities.

Though the nature of the work in the photos cannot be confirmed, Vietnam expert Carl Thayer told AFP that Hanoi might be trying to "stock up" ahead of a code of conduct agreement between various claimants in the South China Sea expected next year.

"If we are moving toward some diplomatic end game at some point in the next year or two, then Vietnam looks like it's trying to get as much as it can before it's prohibited," said Thayer, a professor at the University of New South Wales.

Beijing on Friday delivered a customary rebuke to Vietnam over the apparent dredging, saying it had "indisputable sovereignty" over all Spratly Islands, including Ladd Reef, and surrounding waters.

It urged Hanoi to "refrain from complicating the situation and work with China to safeguard peace and stability in the South China Sea," foreign ministry spokesman Lu Kang said at a regular briefing.

The two countries have long traded barbs over disputed territory in the sea. In 2014 China moved a controversial oil rig into contested waters, prompting riots in Vietnam.

Tensions have eased slightly in recent months but the issue remains incendiary on both sides.

The sea is also claimed by the Philippines, Brunei, Malaysia and Taiwan and is rich in energy reserves, fishery resources and is a busy shipping route.