(CNN) — The US Navy reconnaissance jet flies at 21,500 feet over the South China Sea, 30 miles from the contested Paracel Islands, a group of about 130 small atolls, the biggest of which are home to Chinese military bases.

A voice, saying it’s coming from a People’s Liberation Army (PLA) airport, crackles over the radio of the US Navy P-8 Poseidon as a CNN crew, given rare access aboard the US flight, listens in.

“American aircraft. Chinese airspace is 12 nautical miles. Not approaching any more or you bear all responsibility,” it says.

In a few minutes, a Chinese fighter jet armed with air-to-air missiles intercepts the US plane, nestling in just 500 feet off its port side.

The Chinese fighter jet was so close, the CNN crew could see the pilots turning their heads to look at them — and could make out the red star on the tail fins and the missiles it was armed with.

Lt. Nikki Slaughter, the pilot of the American plane, hails the twin-seat, twin-engine PLA aircraft.

“PLA fighter aircraft, this is US Navy P-8A … I have you off my left wing and I intend to proceed to the west. I request that you do the same, over.”

There’s no reply from the Chinese fighter jet, which escorted the US plane for 15 minutes before turning away.

To a CNN crew aboard the American jet, it’s stark evidence of the tensions brewing in the South China Sea, and between the US and China.

The commander of this US Navy mission has a different take.

“I’d say its another Friday afternoon in the South China Sea,” Navy Cmdr. Marc Hines tells the CNN crew.

Potential flashpoint

Over the past several years, the South China Sea has emerged as a major potential flashpoint in the Asia Pacific. Islands in it, like the Paracels near which the US Navy plane was intercepted Friday, are the subject of overlapping territorial claims in part from China, the Philippines, Vietnam, Malaysia, Brunei and Taiwan.

Not only does the strategic waterway hold vast resources of fish, oil and gas, but about a third of global shipping passes through it — worth about $3.4 trillion in 2016, according to the Center for Strategic and International Studies’ (CSIS) China Power Project.

China claims historic jurisdiction over almost the entirety of the vast sea, and since 2014 has built up tiny reefs and sandbars into artificial islands heavily fortified with missiles, runways and weapons systems — sparking outcry from the other claimants.

The Paracel Islands, called the Xisha Islands by China, are in the northern part of the South China Sea, east of Da Nang, Vietnam, and south of China’s Hainan Island.

Named by 16th century Portuguese mapmakers, they have no indigenous population to speak of, only Chinese military garrisons amounting to 1,400 people, according to the CIA Factbook.

Surrounding them is 12 nautical miles of airspace that China was claiming as its own Friday — a claim Washington doesn’t recognize.

Far to the southeast sits the Spratly Islands chain, just 186 miles from the Philippine island of Palawan.

In 2016, in a case brought by the Philippines, an international tribunal in the Hague ruled that China’s claim to historic rights to the bulk of the sea had no legal basis.

But Beijing has rejected the tribunal’s ruling and continued its military buildup, building bases in the Spratlys, which it calls the Nansha Islands.

China also conducts regular military exercises in much of the South China Sea and maintains a large presence of coast guard and fishing vessels in the disputed waters — which has frequently stoked tensions with its neighbors.

On Friday, while flying close to the Philippines, the US Navy P-8 spotted a PLA Navy guided-missile destroyer and descended to around 1,000 feet to get a closer look — bringing more warnings from the PLA.

“US aircraft. US aircraft. This is Chinese naval warship 173. You are approaching to me at low altitude. State your intention over,” a voice comes over the US plane’s radio.

PLA warship 173 is the destroyer Changsha, likely armed with dozens of surface-to-air missiles.

The US plane will keep a safe distance, its pilot, Lt. Slaughter, replies.

“US aircraft. US aircraft. This is Chinese naval warship 173. You are clearly endangering my safety. You are clearly endangering my safety,” the Chinese ship says.

“I am a United States military aircraft. I will maintain a safe distance from your unit,” Slaughter replies, and the US mission continues.

The US Navy says these missions are routine.

US vessels and aircraft operate regularly where international law allows, the Pentagon says. But China claims the US presence in the South China Sea is what’s fueling the tensions.

When a US guided-missile cruiser steamed near the Spratly Islands in November, the PLA said such action “seriously infringes on China’s sovereignty and security” and is “hard proof is that the US is seeking maritime hegemony and militarizing the South China Sea.”

The US Navy said the US cruiser conducted the operation “in accordance with international law and then continued on to conduct normal operations in waters where high seas freedoms apply.”

For Hines, the US commander of Friday’s mission, the tensions are always less when he’s talking with the Chinese side.

Silence brings uncertainty, he says.

“Whenever there’s no response, it leaves questions. Do they understand what were saying? Do they understand our intentions? Do they understand we don’t mean any harm?” he says.

For the most part Friday, the answers were there. And the encounters were “professional,” Hines says. And he wants to keep it that way.

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