The annual "Han Kuang" exercises, which are being held at the same time as civilian defence drills, will run from July 9-18 and feature newly delivered US high-tech rocket systems.

The self-ruled island democracy faces the constant threat of an invasion by China, which claims Taiwan as part of its territory and has threatened to use force to seize it.

Chinese military pressure has intensified in recent years, with Beijing deploying fighter jets and warships around the island on a near-daily basis.

In the lead-up to the drills Wednesday, Taiwan detected 31 sorties by Chinese military aircraft and seven warships around the island in the 24 hours to 6:00 am (2200 GMT Tuesday), according to the defence ministry.

Taiwan has boosted defence spending and acquired smaller and more nimble weaponry, including drones, to enable its military to wage asymmetric warfare against its more powerful foe.

Twenty-two thousand reservists -- the largest ever call-up -- are participating in this year's drills, having begun a training programme Saturday.

Reservist training includes familiarisation with rifles, squad and platoon machine guns, and tactical manoeuvres.

"Because these reservists have been discharged and away for a long time, upon return they need to undergo specialty refresher training," said an army officer who asked not to be named.

On Wednesday, reservists wearing camouflage uniforms, helmets and boots were put through their paces, practising cleaning, assembling and aiming 65K2 rifles and machine guns.

Taipei is eager to show the world, especially its key security backer Washington, that it is serious about boosting its military capability.

The drills will "let the international community know that we are determined to defend ourselves, and to pass on to China that the nation's military has the confidence and ability to defend a free and democratic life", Taiwanese Defence Minister Wellington Koo said recently.

This year's drills have been extended to 10 days and nine nights, from five days and four nights last year.

- 'Grey zone' scenario -

The Han Kuang began in 1984 when the island was still under martial law.

Troops will simulate various scenarios including "grey zone harassment" -- tactics that fall short of an act of war -- as well as "long-range precision strikes" to combat a Chinese invasion in 2027, defence officials have said.

Officials in the United States -- Taipei's biggest arms supplier -- have previously cited 2027 as a possible timeline for a Chinese invasion of Taiwan.

Recently delivered High Mobility Artillery Rocket Systems (HIMARS) from the United States will be used during the drills.

There will also be a separate live fire event involving US-made advanced M1A2 Abrams tanks.

Taiwanese defence officials have been closely monitoring the war in Ukraine and their use of a decentralised command and control structure.

"When we think practically about combat, we consider what kind of scenario Taiwan might face," a senior defence official said on the eve of the drills.

"Commanders at all levels need to be able to decide what to do based on their understanding of their superior's intent. This usually requires peacetime training to build up their awareness in this regard."

The exercises are being held as President Lai Ching-te, who is a staunch defender of Taiwan's sovereignty, tours the island delivering speeches aimed at "uniting the country".

China has carried out several large-scale military drills around Taiwan since Lai took office last year, as the island rejects China's sovereignty stance.

China specialists at risk analysis firm Eurasia Group said Beijing was "likely" to carry out more military exercises at the end of July.