Hong Kong(CNN) China launched a series of ballistic missiles into the South China Sea this week, according to United States defense officials, part of a flurry of military exercises extending thousands of miles along the country's coastline, as tensions with Washington over the disputed waterway continue to escalate.
Beijing claims almost all of the vast South China Sea as its sovereign territory and has stepped-up efforts to assert its dominance over the resource-rich waters in recent years, transforming a string of obscure reefs and atolls into heavily fortified man-made islands and increasing its naval activity in the region.
China's territorial ambitions are contested by at least five other countries, and have been rejected outright by Washington which has declared Beijing's claims to be illegal under international law.
A US defense official told CNN that the Chinese military launched four medium-range missiles from mainland China on Wednesday. The missiles impacted in the northern reaches of the South China Sea between Hainan Island and the Paracel Islands, known as the Xisha Islands in China, the official said.
In a statement Thursday, the Pentagon described the drills as the latest in a long string of Chinese actions intended to "assert unlawful maritime claims" that disadvantage neighboring countries. The comments follow the announcement Wednesday that the US government will impose sanctions on dozens of Chinese companies for assisting Beijing in the development and militarization of artificial islands in the South China Sea.
Senior Col. Wu Qian, a spokesman for the Chinese Ministry of National Defense, said on Thursday that China had carried out drills in waters and airspace between Qingdao in northeastern China and the disputed Spratly islands -- known as Nansha in China -- in the South China Sea, but did not mention the missiles outright.
According to Wu, the drills "did not target any country."
Though China's Defense Ministry has not confirmed the missile tests, China's government controlled media made several detailed references to the launches, citing reports in overseas media.
Those reports said the missiles involved were DF-21D and DF-26 missiles, both of which have been touted in Chinese propaganda as highly accurate and able to hit ships moving at sea.
"China's DF-26 and DF-21D are the world's first ballistic missiles capable of targeting large and medium-sized vessels, earning them the title of 'aircraft carrier killers,'" the state-run Global Times said on Thursday, citing military observers.
A separate editorial in the same outlet acknowledged speculation around the launch of the DF-21D and DF-26 missiles, saying only that the "Chinese side has neither confirmed nor denied it."
The editorial added that China "must increase its actions in the waters accordingly to suppress US arrogance and reinforce the US understanding that China does not fear a war."
Home to vital international shipping lanes, the South China Sea is widely deemed as a potential flashpoint for a military conflict between the US and China.
Wednesday's tests come a month after two US aircraft carrier strike groups, led by the USS Nimitz and USS Ronald Reagan, completed combined exercises in the South China Sea for the first time in six years.
The US has increased its naval activity in the region in recent months, carrying out routine patrols, referred to as freedom of navigation operations. On Thursday a US guided-missile destroyer sailed near the Chinese-claimed Paracel Islands.
In a news conference call on Thursday, US Vice Adm. Scott Conn, commander of the US Navy's Third Fleet, talked up the US naval presence in the region and its ability to respond to Chinese threats.
"In terms of launching of the ballistic missiles, the US Navy has 38 ships underway today in the Indo-Pacific region, including the South China Sea, and we continue to fly and sail and operate anywhere international law allows to demonstrate our commitment to a free and open Indo-Pacific and reassure our allies and partners," he said.
China's drills, while intended to send a message to adversaries, also offer a rare opportunity for observers to assess the country's advanced military capabilities.
According to Carl Schuster, a retired US Navy captain and former director of operations at the US Pacific Command's Joint Intelligence Center, Wednesday's missile tests showed a high level of sophistication, owing to the involvement of two separate military branches, the People's Liberation Army Navy (PLAN) and its Strategic Rocket Force (SRF).
"This missile shot indicates China has or is very close to establishing procedures for coordinated fleet-SRF anti-ship ballistic missile attacks," he said. That echoes comments made in Chinese state media that Beijing had developed what it termed a "complete system," using aircraft, satellites and ships at sea to monitor the movements of enemy vessels and relay information to the missiles so they can adjust their trajectories during their final attack phase.
Schuster also noted the missiles were fired into an area where Chinese naval vessels were likely operating, indicating a high degree of confidence in the accuracy of the missiles.
More maneuvers are expected in the days ahead. Beijing announced new exercises are set to begin in the Yellow Sea on Saturday and stretch through next Thursday.
Those follow at least four exercises that were underway on Tuesday, when Beijing says a US U-2 spy plane encroached on an exercise off its northern coast.
"The trespass severely affected China's normal exercises and training activities, and violated the rules of behavior for air and maritime safety between China and the United States, as well as relevant international practices," Wu, the Defense Ministry spokesperson, said.
A statement from US Pacific Air Forces to CNN confirmed a U-2 flight -- but said it did not violate any rules.
"A U-2 sortie was conducted in the Indo-Pacific area of operations and within the accepted international rules and regulations governing aircraft flights. Pacific Air Forces personnel will continue to fly and operate anywhere international law allows, at the time and tempo of our choosing," the statement said.