China conducted military drills on Monday, deploying fighter jets and warships to encircle Taiwan as a "stern warning" to the island's "separatist" forces.

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 just over two years.

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

Taiwanese leader President Lai Ching-te, who took office in May, has been more outspoken than his predecessor Tsai Ing-wen in defending the island's sovereignty, angering Beijing, which calls him a "separatist."

Lai vowed on Monday to "protect democratic Taiwan and safeguard national security," while the Defense Ministry said it dispatched "appropriate forces" in response to the drills.

Chinese Foreign Ministry spokesperson Mao Ning hit back, saying "Taiwan independence and peace in the Taiwan Strait are irreconcilable."

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, spokesman 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.

The drills are "focusing on subjects of sea-air combat-readiness patrol, blockade on key ports and areas," Li said.

They also practiced an "assault on maritime and ground targets."

The Liaoning aircraft carrier group "with its troops of army, navy, air force and rocket force" was also involved, Li said.

The previous large-scale drills held in May, three days after Lai's inauguration, were called "Joint Sword-2024A" and lasted two days.