Japan stepped up its military surveillance capabilities on its southern and western island chains over the weekend, in a bid to counter China on a territorial dispute over a group of islets in the East China Sea.
Tokyo's moves, which took place just days ahead of a visit by US President Barack Obama, were perceived by experts as a bid by Japan to push for Washington's support over its claim for sovereignty.
Japan's Self-Defense Force launched a squadron of four E-2C early warning planes from its air base in Naha on Okinawa Sunday, the Jiji and Kyodo news agencies reported.
This is the first time such planes have been based on the island. At the inauguration ceremony in Naha, Japanese Defense Minister Itsunori Onodera said Japan faced a "dangerous situation" as China's continual attempts to "change the status quo by force and threaten the rule of law could trigger emergencies," Kyodo News reported.
"The squadron was newly established to firmly defend our country's territorial land, sea and air," he told reporters, according to Jiji Press.
Japan scrambled fighter jets against approaching Chinese aircraft 415 times in the 12 months ending March 2014, up from 306 times in the preceding year, Jiji said citing defense ministry statistics.
On Saturday, a ceremony was held to start building a radar surveillance unit on Yonaguni, Japan's westernmost island.
Onodera, who attended a ceremony to mark the start of construction, suggested the military presence could be enlarged to outlying islands in the seas southwest of Japan's main islands.
"This is the first deployment since the US returned Okinawa [in 1972] and the calls for us to be more on guard are growing," Onodera told reporters.
About 150 personnel will be deployed at the radar unit by the end of March 2016.
Hu Lingyuan, a professor of Japanese studies at Fudan University, said Japan has been extremely concerned about the People's Liberation Army (PLA) navy's drills in the Western Pacific, a step to realize China's blue-water ambitions.
"Tokyo's frequent military deployments lately suggested its attempts to contain China's naval development, preventing it from going beyond its offshore waters. Even though it fails in reaching the target, the surveillance could still help gather information about the PLA navy's movements, so as to better cope with the situation," Hu told the Global Times.
The military radar station on Yonaguni, part of a long-standing plan to improve defense and surveillance, gives Japan a lookout just 150 kilometers from the disputed Diaoyu Islands.
Tokyo's "nationalization" of the islets in September 2012 led to souring ties with China.
Building the base could extend Japanese monitoring to the Chinese mainland and track Chinese ships and aircraft circling the disputed islets.
Zhang Zhaozhong, a military expert at the PLA National Defense University, told the Global Times that even though the surveillance system set up around waters of the Diaoyu Islands wouldn't effectively pose a threat to China, it is critical to Japan's military buildup.
"Japan used to deploy Aegis-class vessels by hyping up the threat of North Korean missiles. Now, it is using the China threat as an excuse to expand its military deployment in the west and south and build up its defenses," Zhang said.
The bolstering of surveillance comes several days before Obama lands in Tokyo on Wednesday for a summit with Japanese Prime Minister Shinzo Abe, the first state visit by a US president in 18 years.
The US, which under its security pact with Tokyo has pledged to defend Japanese territory, has warned China about taking any action over the Diaoyu Islands, but has not formally recognized Japan's claim of sovereignty over the territory.
Hu said the moves by Tokyo before Obama's arrival are meant to exert pressure on Washington.
"Japan hopes Obama will issue a joint statement with it to clarify the US stance over Sino-Japanese territorial disputes and support Japan's claim of sovereignty over the islets," he said.
Agencies contributed to this story