China's sole operational aircraft carrier passed through the Taiwan Strait Wednesday, the island's defence ministry said, as Beijing steps up pressure on its democratic rival.

It comes weeks after Taiwan's President Tsai Ing-wen warned against what she called China's "military expansion" -- the increase of air and naval drills around the island since she came to power in May 2016.Beijing views the self-ruling island as part of its territory, to be reunified at some point.

Cross-strait relations have become increasingly frosty as Tsai refuses to acknowledge Taiwan is part of "one China".

The aircraft carrier -- a second-hand Soviet ship -- caused a stir in Taiwan when it first entered the strait in January last year, viewed as a symbolic show of strength by Beijing.

The defence ministry said the Liaoning carrier and accompanying vessels entered Taiwan's air defence zone early Wednesday morning and left by noon.

The fleet, which earlier this month had sailed south through the strait that separates Taiwan and China, was seen again heading north, it said.

As in the past two crossings, the ship did not enter Taiwanese waters, the ministry added.

"The military conducted surveillance and took responsive measures during the whole process," the ministry said in a statement.

"There were no unusual activities during the time the fleet passed through the Taiwan Strait. We urge people not to worry," it added.

Built nearly 30 years ago, the carrier was commissioned in 2012 after extensive refits.

According to Taiwan's defence ministry, Chinese warplanes conducted 25 drills around Taiwan between August 2016 and mid-December last year.

The latest Liaoning appearance also comes after tensions were further fuelled by China starting new flight routes in the Taiwan Strait without consulting Taiwan.

Taipei questioned the move as politically motivated and called it reckless, saying it threatened the island's security and could compromise flight safety.