WASHINGTON — Increased interactions between the Chinese and U.S. navies in the contested South China Sea risk becoming made more complicated by the increasingly sophisticated missiles being carried by submarines.

A new report to the U.S. Congress assessing a Chinese submarine-launched missile known as the YJ-18 highlights the danger, noting the missile accelerates to supersonic speed just before hitting its target, making it harder for a crew to defend their ship.

Defence chiefs from several countries in Southeast Asia have warned in recent months of the danger of undersea “clutter” as countries build up submarine fleets and the U.S. challenges China over its claim to a large swath of the South China Sea. This week’s U.S. patrol inside the 12-nautical mile zone that China claims around its man-made islands in the waters saw the USS Lassen shadowed by two Chinese naval vessels.

The YJ-18 missile can cruise at about 600 miles an hour, or just under the speed of sound, only a few metres above the surface of the sea and then, about 20 nautical miles from its target, accelerate to as much as three times the speed of sound, according to an Oct. 28 report from the U.S.-China Economic and Security Review Commission.

“The supersonic speed makes it harder to hit with on-board guns,” according to commission Chairman Larry Wortzel. “It also makes it a faster target for radars.”

The YJ-18’s speed and long range, as well as its wide deployment “could have serious implications for the ability of U.S. Navy surface ships to operate freely in the Western Pacific” in the event of a conflict, the commission found.

Its report came just days after the U.S. warship entered the 12-mile zone around the reefs that China’s turned into man-made islands, one of which may soon be equipped with an airstrip capable of handling the military’s largest aircraft. By passing so close, the U.S. was showing it doesn’t recognize that the feature qualifies for a territorial zone under international law.

Provocative actions by the U.S. may bring serious tensions between the two militaries and may even result in skirmishes, China’s navy commander Wu Shengli told U.S. Chief of Naval Operations John Richardson on a conference call Thursday, according to the Chinese navy’s microblog.

The waters are a vital thoroughfare for the global economy, hosting $5 trillion of international shipping a year. China claims more than 80 per cent of the sea, vying with five rival claimants, including Vietnam and Philippines.

The Office of Naval Intelligence said in its April report on the People’s Liberation Army Navy that China had started to deploy its newest missile, but didn’t give precise details on its range. The Commission said the YJ-18 can travel about 290 nautical miles, more than 14 times as much as its predecessor, the YJ-82. It cited media reports and other unclassified sources.

The report does however contain some potentially good news for U.S. naval commanders.

China’s command and control infrastructure may be insufficient to generate the targeting information needed to take advantage of the YJ-18’s range. Also, the command and control system itself may be vulnerable to countermeasures such as electromagnetic warfare operations, making it difficult for the PLA to track ships and employ the missiles.

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The YJ-18 should not be confused with the so-called “carrier killer” DF-21 anti-ship ballistic missile, which was paraded during China’s commemorations to mark the 70th anniversary of Japan’s defeat in World War II. The DF-21, which would target aircraft carriers, would be fired from land-based mobile launchers. The YJ-18 could impede the progress of a carrier group, the commission report said.

The commission was created by the U.S. Congress in 2000 to investigate and submit an annual report on the national security implications of trade with China.