(Recasts, adds details)
By Ben Blanchard and Yimou Lee
TAIPEI, Oct 4 (Reuters) - Taiwan reported the largest ever incursion by China's air force into its air defence zone on Monday at 52 aircraft, saying the Taiwanese air force had to scramble for the fourth straight day to warn off the intruders, as tensions spiralled.
Chinese-claimed Taiwan has complained for a year or more of repeated missions by China's air force near the democratically governed island, often in the southwestern part of its air defence zone close to the Taiwan-controlled Pratas Islands.
But since Friday, when China marked its national day, the country has embarked upon a massive ramping up of its missions, with almost 150 aircraft flying into the defence zone over the space of four days.
The latest mission included 34 J-16 fighters and 12 nuclear-capable H-6 bombers, which all flew in an area in the vicinity of the Pratas Islands, according to a map provided by the ministry.
Taiwanese fighter jets scrambled to warn away the Chinese planes, while missile systems were deployed to monitor them, it added.
The United States on Sunday urged China to stop its "provocative" military activities near Taiwan, while the island's government has also condemned Beijing.
China has yet to comment. Calls to China's Defence Ministry on Monday went unanswered. The country is in the middle of the week-long National Day holiday.
Taiwan has termed China's activities as "grey zone" warfare, designed to wear down Taiwanese forces and test their abilities.
A Taiwan-based source familiar with security matters in the region told Reuters earlier on Monday the Chinese planes were possibly conducting simulated attacks on U.S. carrier fleets, drills the source said that China has repeatedly carried out near Taiwan in the past few months.
The U.S. Navy referred questions to the Pentagon, which did not immediately respond to a request for comment.
The carrier the USS Ronald Reagan entered the South China Sea late last month. On Monday Japan's defence ministry said that ship, along with another carrier the USS Carl Vinson, had carried out multinational joint exercises in waters southwest of Okinawa, though it gave no exact location.
Okinawa is home to a major U.S. military base and lies just to the northeast of Taiwan.
The start of China's activities might also be deliberately timed as "intimidation" before Japan unveiled a cabinet under new prime minister Fumio Kishida, the source added. Kishida unveiled his Cabinet earlier on Monday.
A Japanese defence ministry official declined to comment.
"Although we watch developments between China and Taiwan closely, we do not comment on each one of them," the official told Reuters.
China has previously said such flights were to protect its sovereignty and aimed against "collusion" between Taiwan and the United States, the island's most important international backer and arms supplier.
China has stepped up military and political pressure to try to force Taiwan to accept Chinese sovereignty.
Taiwan says it is an independent country and will defend its freedom and democracy. (Reporting by Ben Blanchard and Yimou Lee; Additional reporting by Kiyoshi Takenaka in Tokyo; Editing by Toby Chopra, Alex Richardson and Andrew Heavens)
Our Standards: The Thomson Reuters Trust Principles.