US Secretary of State John Kerry's upcoming weekend trip to Beijing is not shaping up to be a relaxing one.

At issue are territorial disputes over the Spratly Islands, in the South China Sea.

According to the Pentagon's annual report on Chinese military strategy, released last Friday, China has used dredging technology to reclaim land in the disputed waters, dumping sediment to expand existing islands or create new ones on top of coral reefs. In fact, China is pursuing land reclamation so quickly that US officials said an additional 3,700 acres of land had been reclaimed since the end of December 2014 when the annual reporting period ended.

On Wednesday The Wall Street Journal reported that Defense Secretary Ash Carter has asked staff to investigate the possibility of sending naval ships and spy aircraft into the proximity of Chinese bases. "It's important that everyone in the region have a clear understanding of exactly what China is doing," a U.S. official told the Journal.

A recent weeklong patrolling visit to the area by the USS Fort Worth attracted the attention of Chinese warships, and according to CNN, the Navy released a photo that showed the Yancheng, a Chinese guided-missile frigate, following the Fort Worth. Officials downplayed the event as typical, but it does come at a time the US is considering sending more ships.

Six countries lay claim to all or part of the South China Sea — China, Malaysia, Brunei, Vietnam, Taiwan, and the Philippines — and all have been scrambling to improve their claim over the Spratly Islands.

Traditionally, countries respect a maritime boundary and do not go closer than 12 miles from the shore of a country's sovereign land. According to Wall Street Journal reports, US Naval command is considering sending ships and spy planes to within 12 miles of several Chinese artificial islands. The United States has made clear that diplomatically it does not recognize China's territorial claims, but such an action would demonstrate real-world disregard for China's claimed maritime borders.

"We are deeply concerned about the US remarks," China's Foreign Ministry spokeswoman Hua Chunying told reporters at a press conference on Wednesday, adding that the US "must make clarification on this."

Professor Malcolm Davis, an expert on Chinese defense policy from Bond University, told VICE News that China's territorial claims aren't generally recognized.

"No one recognizes it except the Chinese, but they've got the clout to enforce it," he said. "If it's allowed to carry on, it sets a dangerous precedent. It's similar to what Russia is attempting in the Ukraine but done with less brute force, in a much more gradual manner."

China also reacted strongly to the Pentagon's report last week. "The US Defense Department's report on China's military and security development situation distorts facts and continues to play up the 'China military threat' cliché," Chinese military spokesman Geng Yansheng said on Saturday, according to the official government Xinhua News Agency.

The Pentagon had claimed the Chinese Peoples Liberation Army (PLA) is in the midst of major buildup and modernization "to improve its armed forces' capacity to fight short-duration, high intensity regional conflicts."

"If there's a rock protruding from the water then it can become an economic exclusion zone, which entitles the claimant to control its resources," Davis said. "This is China's principle motivation for creating these islands."

Other interested parties have also built on the Spratly Islands but none so aggressively as China.

"The weight of the evidence suggests that the Spratly Islands is something that China is willing to go to war over," Davis added.

Regional neighbors have become increasingly concerned, most notably the Philippines, which has accused China of effectively implementing a no-fly-zone over its territory. Filipino Defense Secretary Voltaire Gazmin said on Friday that Filipino aircraft had been contacted by Chinese forces near the Zamora reef on six occasions and told to leave airspace over the island of Pag-Asa, which is inhabited by a small Filipino population and home to a modest garrison of armed forces.

"This is cause for concern because the practice seems made by China although there has been no formal ADIZ [Air Defense Identification Zone] declaration," he said.

The chief of the Armed Forces of the Philippines (AFP), General Gregorio Pio Catapang Jr., visited the island of Pag-Asa this week to demonstrate the commitment of the country to defend its territorial claims against potential Chinese incursion.

"I'm visiting this place to establish the fact that Pag-Asa is a municipality of Palawan, and Palawan is a province of the Republic of the Philippines. Therefore, Pag-Asa is a territory of the Republic of the Philippines," he said to local reporters. He said Filipino aircraft had recently been told by the Chinese military that they were flying in a "military security area."

Catapang expressed concern over Chinese land reclamation, and cited the Subi reef that lays close to Pag-Asa, which is being rapidly converted into an island by China. "Before we landed we saw the reclamation in the Subi Reef and it's really enormous," he said according to local media.

This weekend in Beijing, Kerry will be discussing not only President Xi Jinpeng's upcoming trip to the US this fall, as planned, but the right of nations to freely navigate the South China Sea. A senior State Department official said Kerry will make clear that there's "absolutely no doubt" about the US commitment to that right, Reuters reported.