Manila illegally seized 11 Chinese fishermen and a boat from waters off China's Half Moon Shoal in the Nansha Islands, a move likely to escalate tension in the South China Sea.

Maritime experts warned the Philippines about its act of "pure belligerence" at the Chinese shoal "whose sovereignty is not disputed".

Noel Vargas, head of the Philippine National Police maritime group, confirmed on Wednesday that the maritime police seized the Chinese fishing boat off Half Moon Shoal on Tuesday.

Foreign Ministry spokeswoman Hua Chunying told reporters in Beijing that "China's Foreign Ministry and China's ambassador to the Philippines have made representations to the Philippines, demanding that it provide a rational explanation and immediately release the people and the vessel".

Chinese policy advisors stated that it is necessary for Beijing to "take more resolute measures" to protect territorial sovereignty and dismiss the misplaced illusions of those who want to cause trouble in the South China Sea, including the United States.

An earlier Xinhua News Agency report said several armed men "forced themselves onto the boat and fired four or five shots in the air. They then took control of the boat".

A second Chinese fishing boat escaped, but then encountered another armed boat, though it was able to get away, Xinhua added.

Li Jinming, a professor of maritime law and South China Sea studies at Xiamen University, said this was not the first time that the Philippines has challenged China's sovereignty. It also has done so off China's Huangyan Island.

"The seizure is a purely a political provocation, and Manila should be responsible for any consequences," Li said.

Reuters commented that although there are frequent stand-offs in the South China Sea, "the actual detention of Chinese fishermen or the seizure of a boat is rare".

Vargas told reporters that the fishing boat was being towed to the Philippines' southwestern Palawan Island "where appropriate charges will be filed against them".

Jia Duqiang, a Southeast Asian studies expert at the Chinese Academy of Social Sciences, said Manila seems prepared to handle this case under its domestic laws, which will help boost its groundless claim over the shoal.

"The Philippines has staged this incident at a time when Vietnam is bickering with China over the oil and gas below the South China Sea, and it is waiting to see if this incident will cross Beijing's red line," Jia said.

Manila angered China earlier this year by boosting claims on China's Huangyan Island and Ren'ai Reef.

Beijing reacted strongly after the Philippine Department of Foreign Affairs stated on March 30 that it submitted a memorial to the arbitral tribunal that is hearing the case it brought against China under the United Nations Convention on the Law of the Sea in January 2013.

"Manila wants to shift the focus and cushion the blow from Beijing's countermeasures," Li said.

The recent security commitment from Washington is another major driving force behind Manila's challenge to China, experts said.

During US President Barack Obama's visit to the Philippines last month, Washington and Manila signed a decade-long defense agreement to show more tangible US military muscle and honor commitments of sheltering allies in the South China Sea.

The renewed commitment from Obama "obviously has given Manila the guts to act without fear", Jia said.

"Manila is also testing if Washington will truly honor the commitment and support it if a showdown with China goes to a conflict," Jia said.

Beijing should "take more tangible steps to boost controls over the territorial islands in the South China Sea", otherwise the spillover effect of the seizure incident will go out of control, Li said.

zhangyunbi@chinadaily.com.cn