The Taiwanese defence ministry said it also spotted Chinese coast guard vessels and that Beijing's People's Liberation Army (PLA) had restricted the airspace off the Chinese coast.

There has been intense speculation about the possibility of China launching military exercises in response to Lai's trip to the Pacific last week which included stopovers in Hawaii and Guam.

"In response to these actions by the PLA, the MND has initiated combat readiness drills, factoring in enemy threats, weather conditions, and tactical positioning," the Ministry of National Defense in Taipei said in a statement.

It added that military units were on "high" alert and those on outlying islands had "heightened their vigilance".

The ministry said its forces had "identified PLA Eastern, Northern, and Southern Theater Command naval formations, along with Coast Guard vessels, entering areas around the Taiwan Strait and the Western Pacific".

There was no immediate public announcement by the PLA or Chinese state media about increased military activity around Taiwan.

However, a Beijing foreign ministry spokeswoman said China would "firmly defend" its sovereignty, as Taiwan kicked off its drills.

Taiwan regards itself as a sovereign nation with its own government, military, and currency.

But Beijing insists the island is part of its territory and has not ruled out using force to bring it under its control.

Lai spoke with Republican US House Speaker Mike Johnson in Guam on Thursday -- the highest-level US contact the Taiwanese leader had during a week-long trip -- which drew a barrage of criticism from Beijing.

China's foreign ministry warned Taiwan on Friday that "seeking independence with the help of the United States will inevitably hit a wall", and called on Washington to "cease meddling in Taiwan-related affairs".

In response to a question about possible Chinese military drills around Taiwan following his trip, Lai told reporters on Friday that "raising your fists is not as good as opening your hands".

"No matter how many military exercises, warships and aircraft China sends to coerce neighbouring countries, it cannot win the respect of any country," Lai said.

- 'Unusual movements' -

Taiwan's coast guard said on Monday it had detected "unusual movements" of seven Chinese coast guard ships since early Friday -- the day Lai returned to Taipei.

The Beijing army's airspace restrictions east of Zhejiang and Fujian provinces were expected to be in place until Wednesday, Taiwan's defence ministry said.

"Any unilateral, irrational acts of provocation could severely undermine peace and stability in the Indo-Pacific region and are unwelcome by the international community," the ministry said.

It added that the PLA's "recent activities near Japan, South Korea, the Philippines, and Taiwan have introduced risks and uncertainties to regional security".

Japan's defence ministry said Monday it had detected over the weekend a Chinese navy missile destroyer, frigate and "information-gathering ship" sailing southeast between Okinawa island and Miyako islands towards the Pacific Ocean.

"From the limited information available, the seven restricted airspaces are likely being used for two main purposes: missile testing and simulating no-fly zones, which represent a blocked airspace state," Su Tzu-yun, a military expert at the Institute for National Defense and Security Research in Taipei, told AFP.

Su said the drills "appear politically motivated" and would backfire on Beijing by reinforcing the "China threat theory".

Taiwan faces the constant threat of a military attack by China and relies heavily on US arms sales to boost its defences.

On the eve of Lai's Pacific tour, the United States approved a proposed sale to Taiwan of spare parts for F-16s and radar systems, as well as communications equipment, in deals valued at $385 million in total.

Lai said in the US state of Hawaii during his trip there was a need to "fight together to prevent war", warning there were "no winners" from conflict.

China has launched two large-scale military drills around Taiwan since Lai took office and regularly deploys fighter jets and navy ships to press its claims over the island.

Lai has been more outspoken than his predecessor Tsai Ing-wen in defending Taiwan's sovereignty, angering Beijing which calls him a "separatist".

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MND - MONTAGNE & NEIGE DEVELOPPEMENT