The US Navy is strengthening its presence in the South China Sea by deploying two Independence-class ships specialising in nearshore operations.
This deployment of the littoral combat ships suggested that the US strategy had switched from reconnaissance and deterrence to increasing its attack capability, a Beijing-based think tank said.
The USS Gabrielle Giffords left Singapore's Changi naval base on a mission on November 15, according to ship tracking information, while the USS Montgomery conducted a joint operation with two Australian warships between November 6 and 12.
Both ships were active in the South China Sea, one of the world's busiest shipping routes, where China's claims over the waters conflict with those of a number of neighbouring countries.
Beijing's efforts to expand its military presence in the region by building military outposts with airfields, radars, missile positions and naval harbours on its artificial islands have been challenged by the United States, which has frequently sent warships past the disputed reefs in what it describes as "freedom of navigation" operations.
US Secretary of Defence Mark Esper said on Tuesday that the US was conducting more patrols in the South China Sea to send a signal to China.
The US "rejects attempts by any nation to use coercion or intimidation to advance international interests at the expense of others", he said during a visit to the Philippines, one of the rival claimants.
He also urged nations with South China Sea claims to assert their sovereign rights to put China "on the right path".
"The clear signal that we're trying to send is not that we oppose China per se, but that we all stand for international rules and international laws and that we think China should abide by them as well," Esper said.
Most vessels used in previous freedom of navigation missions were guided-missile destroyers or cruisers.
But littoral combat ships have unique advantages in the region, according to a report by the South China Sea Strategic Situation Probing Initiative, a think tank affiliated with Peking University's Institute of Ocean Research.
The report said the ships' low draught gave them better access to shallow waters, which would help the effort to carry out reconnaissance missions in the scattered reefs of the disputed Spratly chain. Their fast speed - up to 50 knots - was also an advantage in freedom of navigation missions, the report said.
Thanks to their modular design, the ships can also quickly switch to carrying out combat missions or anti-mine and anti-submarine operations.
In particular, the USS Gabrielle Giffords, which is equipped with advanced anti-ship missiles, can supplement the operations of the Seventh Fleet.
At the start of October the ship test-fired a naval strike missile with a range of over 100 nautical miles (185km or 115 miles) - the first test of the stealth missile in the Indo-Pacific region.
The ships' deployment represented a subtle change in the US Navy's strategy in the South China Sea, suggesting that commanders were starting to focus on practical ways to improve their strike capability in the region by "proactively seeking military deterrence and preparing for potential military conflicts", the report said.
However, Hong Kong-based military affairs commentator Song Zhongping said the Gabrielle Giffords and Montgomery did not pose a significant threat to Chinese-controlled islands and reefs in the South China Sea because they lacked stealth capabilities and were fragile.
"In response, the Chinese side could strengthen land-based anti-ship missiles and planes or even the deployment of aircraft carrier in the future," Song said.
Song added that the littoral combat ships programme did not seem to be hugely popular with the US Navy, but the maker of those vessels would probably like to sell them to US allies in the region.
"So to deploy them in the region could also be an advertising display," he said.
Additional reporting by Bloomberg
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