Provocative activity in the South China Sea is clearly visible in the fiercely contested Spratly Islands, satellite pictures released today show.

Dredging work appears to be underway on a reef that is covered with water at high tide, but is an important stake in the hotspot, with a Vietnamese flag, lighthouse and soldiers stationed there.

"We can see that, in this environment Vietnam's strategic mistrust is total … and they are rapidly improving their defences," retired British navy intelligence analyst, Trevor Hollingsbee said.

The dredging risks provoking the anger of Vietnam's main rival in the sea — China.

The whole South China Sea was once a region of uncontested traditional fishing waters, but as billions of dollars of gas and oil have been discovered, countries are vying for control.

A trip to the more northern Paracel Islands exposes the rawness of national sensitivities.

Sparkling on the horizon as our boat navigates the swell and leaving Vietnam behind us, the Paracels are one of the world's most contentious flashpoints.

'My ancestors have always fished here'

Captain Bui Ngoc Thanh and his crew of 18 are heading north. He risks harassment in these forays, but there is the lure of abundant fish and giant clams.

The fishermen also have a defiant attitude-fuelled by their country's own historic claims on the area.

The latest attack was only two weeks ago, Captain Bui said.

The Chinese Coastguard stopped them near Woody Island. Officials boarded the boat and beat the crew using clubs and electric cattle prods.

Captain Bui Ngoc Thanh claims his boat and crew were recently boarded and beaten by the Chinese Coast Guard.

"They climbed on my roof and cut the Vietnamese flag, cut our radio antennae and stole all our fish and equipment," Captain Bui said.

"They forced us to lie down on the front deck until they had taken everything."

"They yelled at us and their translator told us that we were fishing illegally in Chinese waters. But I know these are Vietnamese waters because my ancestors have always fished here," he said.

China justifies its claims of sovereignty over much of the South China Sea because of ancient trade routes.

As his men haul in their nets, Captain Bui says fish stocks are down because the subsidised Chinese fishing fleet is expanding.

"In the past we could get the same amount of fish in one week that we get now in one month," he says.

Soon Captain Bui and his crew head back with their catch to Ly Son Island, just off Vietnam's coast.

Nguyen Qua Chinh is the President of the Ly Son Fishermen's Association. ( ABC News: Ben Bohane )

Vietnam building strategic partnerships

Ly Son sits between Vietnam's two main two hotspots - the Paracels in the north claimed by Vietnam and China, and the Spratleys in the south, with overlapping claims by a cluster of different countries.

Ly Son is Vietnam's historic gateway for its military and fishermen to their "East Sea". It is the most northerly island Vietnam still controls.

At a diplomatic level, Vietnam and China seem eager to defuse tensions.

Prime Minister Nguyen Xuan Phuc's six-day visit to China in September culminated in reassurances: "China and Vietnam can … promote maritime cooperation through friendly negotiations," proclaimed China's President Xi Jinping.

But its build-up of military installations on artificial islands continues unabated, with hangars being built to house jet fighters.

Vietnam is making strident moves to counter Chinese domination.

Earlier satellite pictures show an airstrip on Vietnam's Spratly Island recently extended to accommodate most planes in its air force, according to the Centre for Strategic and International Studies in Washington.

In August, Reuters reported Vietnam had discretely fortified five Spratly Islands with rocket launchers within range of China's newly-built airstrips. Chinese state-run media described the move as "a terrible mistake".

"It is within out legitimate right to self-defence to move any of our weapons to any area at any time within our sovereign territory," Deputy Defence Minister Senior Lieutenant-General Nguyen Chi Vinh said.

Vietnam is building strategic partnerships, with India extending a $US500 million line of credit for defence equipment, and Japan has supplied coast guard vessels and joined in naval exercises.

"We share similar strategic concerns and maybe we trust India and Japan more to be longer term allies," said noted authority on the South China Sea, Professor Tran Duc Anh Son.

The monument represents Vietnam's claim to the sea. ( ABC News: Ben Bohane )

Back on Ly Son, I am shown a museum dedicated to Vietnam's claims to the sea.

The legal argument derives from 1836 when the king of Hue began dispatching soldiers once a year to the islands, hunting for pearls, giant clams and salvaged treasure from boats which sank on treacherous reefs. This regal connection is significant.

"Vietnam is the rightful owner of the Paracels and Spratlys," argues Nguyen Lan Anh, Deputy Director of East Sea policy.

"Sovereignty cannot be claimed by fishermen, but it can be claimed through edicts of a king."

Pham Thoai Tuyen, 72, is an 11th-generation descendant of one of the six original families who settled the island in 1609. His forebears placed stone markers on a few Paracel islands, which they named after themselves.

"These islands have been part of Vietnam for a long time," Mr Pham says.

He is angered when he hears of the increasing harassment by Chinese Coast Guard ships and fishermen.

At least 18 boats out of the 250-strong Ly Son fleet have been sunk by the Chinese in the past three or four years, while many others have been rammed and attacked, says Nguyen Quoi Chinh, president of the local fishermen's association.

Some fishermen have been killed.

There is some evidence regional tensions have calmed over the Spratlys since Philippines President Rodrigo Duterte softened his country's stance towards China's claims in August.

Almost alone, Vietnam is firming its stand and the Coast Guard reports increasing hostilities.

So far, this is a slow-burning war between rival Coast Guards and fishermen, as the competing nations deploy coast guards instead of navies to avoid escalation.

At Coast Guard headquarters in Ky Ha harbour, Colonel Tran Van Dung escorts me onto the bridge of one of his newest ships, and explains the challenges.

Coast Guard Colonel Tran Van Dung says rescues of fishermen has increased in recent years. ( ABC News: Ben Bohane )

His fleet of 20 ships must cover 175,000 square kilometres, including the Paracels and northern Spratlys.

"In recent years we have rescued many fishermen and protected them in their traditional fishing areas," he says.

"We need to double our fleet and personnel — and this is happening."

Fishermen are encouraged to operate in the claimed waters as they play an important role in the continuation of sovereignty and provide useful information.

Vietnam cannot win a war against China, but it can hold its ground and repel them, as it has done in battles before — the last was two years ago when China provocatively hauled an oil rig into the contentious waters.

Violent riots in Vietnam saw a number of ethnic Chinese people killed and thousands flee the country, before the rig was moved.

While China resists addressing the basis of the competing claims, underlying problems are not being resolved. Dangerous days are looming, as frictions increase.

Back on his new coast guard cutter, Colonel Tran remains confident.

"We will always be the winner in war because we have the support of our people," he says, evoking past victories Vietnam has won against great powers like the US and France.

He is clearly thinking now of a rising power to the north as he descends the gangway again.