Exclusive: Unseen Battle For South China Sea

Technically, we are in international waters, but we are heading close to the Chinese-controlled Subi Reef, and the Philippines fishermen on board are frightened.

The captain and his crew are up on deck keeping watch, anxiously scanning the horizon.

Some of the men want to turn around. They are afraid of being intercepted, and rammed.

Tensions in the region are high - and rising.

China lays claim to most of this sea as its sovereign territory, but that is disputed by five other rival claimants.

And this is more than a local stand-off.

The US has been carrying out what it calls freedom of navigation patrols - effectively rejecting China's claim to territorial waters, and enforcing international maritime free passage.

:: Why Is There A Dispute In The South China Sea?

The Philippines has taken China to the UN Court of Arbitration over its actions here - the verdict is expected shortly - but Beijing has already rejected the court's jurisdiction.

In the last couple of weeks they have been holding large naval exercises in the area.

"I am afraid," the captain of our boat tells us. We agreed not to use his name. "I order my people to see [look out]."

He says they have been warned off Chinese areas before.

"They say this is China territory, leave from this, this is the China territory. We cannot enter."

His family, and those of every man on board, depend on this boat.

The captain decides we are getting too near and steers away.

From the neighbouring Philippine-held island, we get a closer look at what they are afraid of.

Through our camera lens we can see the scale of the Chinese construction on Subi Reef, which appears to be going on still.

We can see cranes and what appear to be several cement plants.

There is a huge dry dock, and a large dark structure close by - it looks like some sort of vast fortification going up.

For a country that claims to be building search and rescue facilities "for public good purposes" it is certainly building on quite a scale. And at speed.

Satellite images show Subi Reef in 2012, almost entirely submerged.

Now it is a facility of more than three million square metres, with its own airstrip under way, long enough to land any Chinese military aircraft.

One of the Philippines officers stationed nearby, who asked not to be named, says the reef feels like a "knife at their throat", and that they watch it getting bigger every day.

China insists its development here is peaceful and appropriate, that it is merely strengthening its national defences.

The next day we see a Philippines military transport aircraft coming in to land at their base on the disputed island of Pagasa, braking hard and pulling up in a huge cloud of dust.

There is a multi-national power struggle going on out here - involving several countries' armed forces - but because this is all taking place so far out at sea, for the most part it is happening out of sight of the rest of the world.

The sheer distances involved are immense.

It takes a long day, and a very long night at sea, to reach our next location.

From a distance the Sierra Madre looks like a ghost ship, rusting badly and listing to one side.

But this is an armed Philippines military position on this most remote of front lines.

The ship started life as a US tank-carrier in the Second World War, before being transferred to the Philippines and deliberately grounded on this reef, known as Second Thomas Shoal, in 1999, to defend their territorial claim.

The Sierra Madre has been manned continuously in the years since.

Some of the marines stationed on board come out to meet us.

Conditions are tough - it is brutally hot, and they are running low on drinking water - but they tell us they feel they are serving their country out here.

If it comes to it, they tell me, they are prepared to lay down their lives in this cause.

"We are gonna die, mam," one young serviceman says. "We cannot run. It's our job."

"It's our territory," his colleague adds. "This is Philippine territory. We will do whatever it costs."

All the while, circling them in the distance, we can see a 3,000-ton, state-of-the-art Chinese coastguard cutter.

Time may be on its side.

Up close, the old warship's bow is splitting apart.

We see crabs scuttling up the side and into the gaping holes.

This is still a commissioned Philippine Navy ship - with the protection of its American ally that invokes, but its guns look like they have seen better days.

It is a striking image - the old rusting outpost, and the gleaming white Chinese coastguard ship - staring each other down.

And so the stand-off out here goes on.

They never communicate, the marines tell us, just watch each other and wait.