China considers Taiwan to be part of its territory and has ramped up military activity around the island in recent years to pressure Taipei into accepting its claims of sovereignty.

"The Liaoning is passing through the Taiwan Strait now, sailing north along the west of the median line (of the passage) and we are closely monitoring it," Defence Minister Wellington Koo told reporters.

The Liaoning, China's oldest aircraft carrier, took part in Beijing's large-scale military drills around Taiwan last week that were condemned by Taipei and its key backer Washington.

A blockade was among the exercises carried out.

Koo warned on Wednesday that an actual blockade of Taiwan would be an "act of war" and have a "very serious impact on the global economy".

China has two aircraft carriers in active service, and a third undergoing sea trials. The Liaoning has previously passed through the strait.

It appeared to be returning to Qingdao port in eastern China via the Pratas Islands, in the northern part of the South China Sea, for "replenishing and necessary maintenance", said Jiang Hsin-biao, a military expert at Taiwan's Institute for National Defense and Security Research.

Its involvement in the recent military drills was for "the purpose of practising against foreign forces and intimidating Taiwan", Jiang told AFP.

- Stronger language -

Beijing sent a record number of military aircraft -- including fighter jets and drones -- as well as warships to encircle Taiwan on October 14 in what Beijing said was a "stern warning to the separatist acts of 'Taiwan Independence' forces".

It was in response to Taiwan President Lai Ching-te's National Day speech on October 10 in which he vowed to "resist annexation", and insisted that Beijing and Taipei were not subordinate to each other.

Lai, who took office in May, has used stronger language than his predecessor Tsai Ing-wen in defending Taiwan's sovereignty, angering China's leaders in Beijing who call him a "separatist".

Taipei said Tuesday the live-fire drill could be part of Beijing's "tactics to bolster its intimidation in conjunction with the dynamics in the Taiwan Strait".

Over the weekend, a US and a Canadian warship passed through the 180-kilometre Taiwan Strait, part of regular passages by Washington and its allies meant to reinforce its status as an international waterway.

Beijing condemned the passage as disrupting "peace and stability" in the strait.

Taiwan's defence ministry said Wednesday it had detected 15 Chinese military aircraft and six navy vessels in the skies and waters around the island in the 24 hours to 6:00 am Wednesday (2200 GMT Tuesday).

China and Taiwan have been ruled separately since 1949 after Chiang Kai-shek's nationalist forces fled to the island following their defeat by Mao Zedong's communist fighters.

While Taiwan has its own government, military, and currency, Beijing insists the island is part of its territory and has refused to rule out the use of force to bring it under its control.