Taipei, Taiwan - China deployed fighter jets and warships to encircle Taiwan on Monday, in drills Beijing said were aimed at sending a "stern warning" to "separatist" forces on the island.

Beijing has not ruled out using force to bring Taiwan under its control and Monday's drills represent its fourth round of large-scale war games in the past two years.

The US said China's actions were "unwarranted" and risk "escalation" as it called on Beijing to act with restraint.

Taiwanese President Lai Ching-te, who took office in May, 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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Lai vowed Monday to "protect democratic Taiwan, and safeguard national security," while the defense ministry said it dispatched "appropriate forces" in response to the drills.

AFP journalists near the Hsinchu air force base, in the north of Taiwan, saw 12 fighter jets take off on Monday.

Outlying islands administered by Taipei were on "heightened alert" and "aircraft and ships will respond to enemy situations in accordance with the engagement rules," Taiwan's defense ministry said.

Beijing said its exercises served as a "stern warning to the separatist acts of 'Taiwan Independence' forces."

The drills, dubbed Joint Sword-2024B, are testing troops' "joint operations capabilities" according to Captain Li Xi, spokesperson for the Chinese military's Eastern Theater Command. They are taking place in "areas to the north, south and east of Taiwan Island," he said, and are "focusing on subjects of sea-air combat-readiness patrol, blockade on key ports and areas." They also practiced an "assault on maritime and ground targets."

Fighter jets and warships had been deployed, Chinese state media said.

China's coast guard was also dispatched to conduct "inspections" around the island. A diagram released by the coast guard showed four fleets encircling Taiwan and moving in an counterclockwise direction around the island.

The coast guard of the eastern province of Fujian – the closest area on the mainland to the island – also said it was conducting "comprehensive law enforcement patrols" in waters near the Taiwan-controlled Matsu islands.

Taiwan said four "formations" of China coast guard ships were patrolling around the island, but they had not entered its prohibited or restricted waters.

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China has ramped up military activity around Taiwan in recent years, sending in warplanes and other military aircraft while its ships maintain a near-constant presence around the island's waters.

"In the face of enemy threats, all officers and soldiers of the country are in full readiness," Taiwan's defense ministry said Monday.

Lai convened a high-level security meeting over the drills, said Joseph Wu, secretary-general of the National Security Council, who described the exercises as "inconsistent with international law."

In his speech Thursday celebrating the island's National Day, Lai vowed to "resist annexation" of Taiwan and insisted Beijing and Taipei were "not subordinate to each other."

Lai's Democratic Progressive Party has long defended the sovereignty and democracy of Taiwan, which has its own government, military, and currency.

Beijing on Monday said the drills were "a legitimate and necessary operation for safeguarding state sovereignty and national unity."

Taiwan's defense ministry said Monday that 25 Chinese aircraft and seven navy vessels were detected around island in the 26 hours to 8:00 AM.

Lieutenant Colonel Fu Zhengnan, a researcher at the Chinese Academy of Military Sciences, said in a video shared by state media that the drills could "switch from training to combat at any time."

"If Taiwan separatists provoke once, the PLA's operation around the island will make their first move," Fu said, referring to China's People's Liberation Army.

Taiwan's coast guard said Monday it had detained a Chinese man on one of its outlying islands after a possible "gray zone intrusion," referring to tactics that fall short of a direct act of war.

During morning rush-hour in Taipei, people appeared to be largely unperturbed by the latest drills.

"I won't panic too much because they quite often have drills," 34-year-old engineer Benjamin Hsiao told AFP. "It's not the first time in recent years anyway, so I feel a bit numb."

The current dispute between China and Taiwan dates back to a civil war in which the nationalist forces of Chiang Kai-shek were defeated by Mao Zedong's communist fighters and fled to Taiwan in 1949.

Since then, China and Taiwan have been ruled separately.