A Japanese submarine visited a former US navy base at Subic Bay in the Philippines on Sunday - the first such visit in 15 years - in a sign of deeper international involvement in territorial disputes in the South China Sea.
The base is just 200 kilometers from China's Huangyan Island, the scene of confrontations between Chinese and Filipino ships in 2012.
With Monday being a national holiday in China, Beijing has yet to respond to the visit.
The submarine's arrival came one day before the Philippines and United States started a massive military drill. The drill scenarios include retaking an island seized by an enemy in the South China Sea, which Reuters reported was "likely to rile China".
Xinhua News Agency said in a commentary on Monday that the exercise "caps Manila's recent attempts to involve outsiders in a regional row".
The 12-day annual Balikatan (shoulder-to-shoulder) exercise will be joined by US allies Australia and Japan for the first time.
The Xinhua commentary said that the exercise, held near disputed waters of the South China Sea, is viewed widely as the latest attempt by the Philippines to demonstrate its military alliance with the US.
The Japanese submarine, one of the newest and largest in the country's navy, was reportedly escorted by two Japanese destroyers on a tour of Southeast Asia.
Captain Hiraoki Yoshino of Japan's Maritime Self-Defense Force told reporters, "We don't have any message to any country", but he described the Philippines as "a very important ally".
Japan's Minister of Defense Gen Nakatani said on Sunday that Tokyo would seek further cooperation with other Asian countries to "ensure stability in the South China Sea".
Nakatani is scheduled to visit the Philippines this month to discuss defense cooperation.
Japan, which has territorial disputes with China in the East China Sea, has been increasing its presence in the South China Sea. This has included sending more ships and planes to countries such as Vietnam and the Philippines, which also have overlapping claims with China.
Japan has offered to lease three TC-90 surveillance planes to the Philippines to help enhance its capability in monitoring the South China Sea. The deal is expected to be sealed this month.
Japan has also agreed to supply radar technology to the Philippines.
Yin Zhuo, director of the People's Liberation Army Navy's Expert Consultation Committee, said Japan's actions in the South China Sea are aimed at partially supporting the US pivot-to-Asia strategy and using that to contain China.
"In addition, it is using the excuse of stability in the South China Sea to legalize its future involvement in the region."
