Tensions in the South China Sea could spiral out of control if the US starts patrolling too close to Chinese islands, with any military confrontation between China and the US escalating to a dangerous level, analysts said Wednesday.

Speaking after a two-day meeting between US and Australian foreign and defense ministers in Boston, US defense secretary Ashton Carter said Tuesday that the US would sail and fly wherever international law allows, including the South China Sea.

His remarks were rebuked by China's foreign ministry, which said China has indisputable sovereignty over certain South China Sea islands and their surrounding waters and that China is not the one that had militarized the region.

"I want to point out that some countries have recently flexed their military muscles again and again in the South China Sea," foreign ministry spokeswoman Hua Chunying told a daily news briefing Wednesday.

"This is the biggest factor in the militarization of the South China Sea. We hope the relevant countries cease hyping up the South China Sea issue and scrupulously abide by their promises not to take sides on the territorial disputes," she said.

Carter's statement came a day after The New York Times reported that the US has been briefing its allies in Asia, including the Philippines, on plans to conduct naval patrols near Chinese islands, which could come as close as within the 12 nautical mile limit.

The patrols look more imminent according to a Wednesday Reuters report, which, by quoting some analysts in Washington, said the patrols could happen at the end of this or next week.

Countermeasures

"What will happen is that China will take necessary countermeasures [if the US begins patrolling the area.] The actual measures will depend on how frequently the US decides to enter the airspace or waters close to the islands and what kind of aircraft or ship they plan to send," Wu Shicun, president of the National Institute for the South China Sea, told the Global Times.

According to Wu, the first measure would involve diplomatic and military warnings. If the situation escalates, China may dispatch planes to tail US aircraft to decide if there is hostile intent. If this is believed to be so, the next step would be for the Chinese military to expel the US ships and planes.

Wu warned that there could be considerable danger, and if further measures need to be taken, the risk of a military clash or even casualties, based on either miscalculation or coincidence, would significantly rise.

"I think the bottom line for both China and the US is to make sure there is no open conflict or casualties," Wu said.

His opinion was echoed by Hu Bo, a professor at the Peking University Ocean Strategy Research Center, who said that both China and the US will remain restrained to prevent any confrontation from evolving into a full-blown war.

"The problem is, both countries need to demonstrate their strong will to the world while trying to keep their heads cool. This makes controlling the situation difficult," Hu said.

Although entering within 12 nautical miles of Chinese islands may not be technically difficult for the US military, analysts believe the important question the US should ask itself is whether it will face a better situation in the South China Sea if it decided to take such action.

"China is unlikely to let the US get away with it. A likely outcome would be a long-term military stand-off in the South China Sea," Hu said.

Civilian use stressed

The US intervention could also change what China plans to do with the South China Sea islands, experts said. 

Chinese President Xi Jinping said last month during his visit to the US that China did not intend to militarize the islands.

Foreign ministry spokeswoman Hua on Wednesday re-iterated that China's purpose of island construction is for civilian use.

She noted that China is only deploying limited military equipment for necessary defensive purposes, which is "understandable given that some countries are flexing their muscles and frequently conduct targeted large-scale military exercises with allies."

"The proportion of military facilities on these islands depends on how much threat the US and its allies exert on China," Wu said.

"If the US military comes within 12 nautical miles of these islands, it would only be more reasonable for China to speed up its construction of military facilities, which at the moment is restrained," Hu said.