Reporter’s notebook: Flying with the US military as it keeps tabs on China over the South China Sea
WATCH: ABC News' Bob Woodruff goes on patrol with the Skinny Dragons to monitor the controversial islands that China has been building on reefs since 2013.

Just off the beach in Okinawa, Japan, at the American Kadena Air Force Base, the Navy Maritime Patrol Squadron Four takes to the sky to watch what China is up to in the South China Sea.

For a couple years now, I have been trying to report on what is changing so quickly in these waters. China is growing, and from thousands of feet up, you can witness the geographical expansion.

Over the past few years, the country has created about 3,200 acres of artificial land by covering ocean reefs with sand and cement, turning them into islands. This enables the government to claim the land as property.

DigitalGlobe overview imagery comparing Fiery Cross Reef from May 31, 2014 to June 3, 2016. Fiery Cross is located in the western part of the Spratly Islands group.

It would be like the United States creating neighboring islands to Hawaii, all throughout the Pacific Ocean.

But the U.S. isn't doing this — China is.

A satellite image of Mischief Reef located in the Spratly Islands in the South China Sea on Feb. 18, 2016.

The U.S. Navy has been strict about giving journalists permission to join these missions ever since China began expanding on the reefs in 2013. ABC News' trip was the third time they allowed journalists to join since then.

ABC News correspondent Bob Woodruff flew with the U.S. Navy Maritime Patrol Squadron Four over the South China Sea on Sep. 6, 2018.

We flew on a Boeing P-8 Poseidon, a high-tech military aircraft used for surveillance missions like this. On board, it's like a 747 but without the usual seats. As we flew over the islands, high-quality cameras mounted on the bottom of the plane recorded video. Crewmembers monitored screens inside the plane, looking for any changes below.

Members of the U.S. Navy Maritime Patrol Squadron Four pilot an aircraft over the South China Sea, on Sep. 6, 2018.

The islands are growing, and the construction isn't only for civilians — there is plenty of military development as well. Last year, most construction was focused on the island named Fiery Cross, where new projects covered 27 acres of land, or about 110,000 square meters, according to the Asia Maritime Transparency Initiative. Subi Reef, another island, saw the development of underground facilities and, notably, the installation of two "elephant cage" antennas for signals intelligence. Meanwhile, new radar and communications displays have also been spotted on the northern side of another island, Mischief Reef.

Satellite imagery shows the Subi Reef in the South China Sea, a part of the Spratly Islands group, on May 28, 2018.

We remained in the sky for about seven hours, and although we were able to shoot with our own cameras through the windows, the islands were very far away. The advanced cameras the Navy uses are far better.

A member of the U.S. Navy Maritime Patrol Squadron Four aboard an aircraft above the South China Sea, on Sep. 6, 2018.
The U.S. Navy uses advanced cameras to monitor construction on islands in the South China Sea, such as on the Subi Reef, as seen in this image taken during a recent flight and provided by the Navy.

In order to burn less fuel, the pilots flew at an altitude of about 20,000 feet. They told us they could fly as low as 200 feet, but that it wouldn't be of much benefit. We also flew about 20 miles from the islands in order to avoid serious confrontation with China's military — a conservative distance, considering international law stipulates we can't fly closer than 12 miles. The crew didn't want to risk it.

An aerial shot of part of Mischief Reef in the disputed Spratly islands, in the SOuth China sea, on April 21, 2017.

Despite this kindness and respect, China did not keep quiet. We heard warnings over the radio six times, both in Chinese and English. Each time it was almost the same message, noting that we were in a "U.S. military aircraft" and cautioning us to "leave immediately." To which the U.S. would respond: "I'm a sovereign United States Naval aircraft conducting lawful military activities here in the international airspace of any coastal state. In exercising these rights as guaranteed by international law, I’m operating in due accord with the rights and duties of our states.”

America is stepping up surveillance of the South China Sea because there is growing fear that China will someday have full control over 1.3 million square miles of blue water.

A member of the U.S. Navy Maritime Patrol Squadron Four radios a response to a warning from Chinese personnel, over the South China Sea, on Sep. 6, 2018.

In addition to over 10 percent of the world's fish catch coming from the South China Sea each year, it's estimated 11 billion barrels of oil and 190 trillion cubic feet of natural gas lie beneath its waters. About one-fifth of the world's trade flows through the South China Sea, making the country's expansion one worth monitoring. And U.S. military forces are certainly watching, even if they're not doing anything to stop it.

ABC News correspondent Bob Woodruff flew with the U.S. military over the South China Sea on Sep. 6, 2018.

Military construction and development on the islands are speeding up, too. China’s president Xi Jinping told U.S. Defense Secretary Jim Mattis in June that “not a single inch of the territory left behind by our ancestors must be lost,” Chinese state broadcaster CCTV reported, according to the Associated Press.

While the U.S. Navy will continue to surveil the South China Sea, their missions will likely only slow the country down. I have been visiting and working in China for over 30 years, and one constant I have always found to be true is China's ability to be patient.

Reporters pose with members of U.S. Navy Maritime Patrol Squadron Four, in Okinawa, Japan, on Sep. 6, 2018.