The ship, which has yet to be named, is Beijing's second carrier and will add massive firepower to its navy once it is officially put into service, as the country faces tensions with self-ruled Taiwan and regional neighbours around the disputed South China Sea.

The carrier crossed the sensitive waters on Sunday before entering the South China Sea for "scientific research tests and routine training", navy spokesman Cheng Dewei said on an official social media account.

Cheng said it was "normal practice" for carriers that are under testing to conduct cross-regional trials.

"It is not aimed at any specific target and has nothing to do with the current situation," Cheng said without elaborating.

The sail-by comes as Taiwan gears up for presidential elections in January.

Foreign minister Joseph Wu tweeted on Sunday that China "intends to intervene" in Taiwan's elections", adding: "Voters won't be intimidated!"

Taiwan's defence ministry said it had dispatched ships and planes to track and closely monitor the carrier's movements, and that US and Japanese vessels trailed it in the strait.

Chinese foreign ministry spokesman Geng Shuang said Monday the naval operation was "nothing to be surprised about" and in line with international law.

China, which sees democratic Taiwan as part of its territory, has stepped up military drills around the island since Beijing-sceptic President Tsai Ing-wen, who is seeking re-election, came to power in 2016.

"We are keeping abreast of the situation in the entire Taiwan Strait," Tsai told reporters on Monday.

"I believe that we are capable of defending ourselves and I ask our countrymen to rest assured."

Tsai -- who has voiced support for Hong Kong's pro-democracy movement -- has described the elections as a fight for Taiwan's freedom and democracy.

Her challenger favours much warmer relations with China.

On the same day as the sail-by, Tsai announced that former premier William Lai, who has styled himself as a "Taiwan independence worker", would be her running mate in a move likely to irritate Beijing.

- 'Undoubtedly deliberate' -

Eric Hundman, assistant professor of political science at NYU Shanghai, said that the ship's crossing was a "continuation of Beijing's consistent efforts to pressure Taipei".

"The choice to sail through the Taiwan Strait was undoubtedly deliberate and probably intended as a signal to both Taiwan and the US of China's increasing naval capabilities," he said.

The "Type 001A" carrier was launched in 2017 and sea trials began the following year. It is expected to be commissioned in 2020.

State television on Monday broadcast images of the Type 001A with at least three fighter jets on its flight deck.

The state-run Global Times cited an anonymous military expert as saying the ship likely sailed through the strait to dock at its possible home base in Sanya, a port in southern China's Hainan island.

Hainan province is in the South China Sea east of Vietnam, which has competing claims in the waterway along with China, Malaysia, the Philippines, Taiwan and Brunei.

"Basing in Sanya would give the carrier a geographically advantageous staging point for operations into the Indian Ocean and, through Bashi Channel south of Taiwan, into the West Philippine Sea," Collin Koh, a research fellow at Singapore's S. Rajaratnam School of International Studies, told AFP.

China has one other carrier -- the Liaoning, a repurposed Soviet carrier bought from Ukraine that went into service in 2012.

A US think-tank reported in May that recent satellite photographs indicated that construction of a third Chinese aircraft carrier was well under way.

Adding a third aircraft carrier will put China in an elite club among naval powers but it will still lag far behind the United States, which has 10 nuclear-powered Nimitz-class "supercarriers" currently in service.