Taiwan's military set up an emergency response centre and raised its alert level on Monday, saying China has set up seven zones of reserved airspace and dispatched naval fleets and coast guard boats to waters around the island.
China, which claims democratically governed Taiwan as its own territory, had been expected to launch another round of exercises in response to Taiwan President Lai Ching-te's trip to the Pacific, which included stopovers in Hawaii and the US territory of Guam, security sources had told Reuters.
China has set up seven "temporary reserved areas" of airspace to the east of its eastern Fujian and Zhejiang provinces, Taiwan's defence ministry said in a statement, adding those zones are valid from Monday to Wednesday.
Such zones are temporarily reserved and allocated for a particular user during a set period, though other flights can pass through with permission from controllers, according to international rules.
China's defence ministry did not immediately respond to a Reuters request for comment.
A Taipei security source told Reuters that China currently has nearly 90 navy and coast guard ships in waters near Taiwan, the southern Japanese islands and the East and South China Seas, of which around two-thirds were navy vessels.
Taiwan's military said it has activated its "combat readiness exercises" at strategic locations and its naval and coast guard boats are closely monitoring the Chinese military activities.
"Any unilateral and irrational, provocative actions could seriously destroy peace and stability in the Indo Pacific and that will not by welcomed by the international community," Taiwan's defence ministry said.
Taiwan's coast guard, in a separate statement, said that seven Chinese coast guard ships have been carrying out "grey-zone harassment" against the island since earlier in the day.
China does not have the right to carry out any intrusion or law enforcement actions in Taiwan's waters, and Taipei has the right to take necessary measures in response, the coast guard said.
Taiwan says China has been using "grey zone" tactics that stop short of actual combat to test and pressure Taiwanese forces, including daily air force and navy missions around the island and regular coast guard patrols.
(Reporting by Ben Blanchard and Yimou Lee in Taipei; Editing by Kim Coghill and Jamie Freed)