A group of Chinese warships led by the country's sole aircraft carrier has entered the top half of the South China Sea, Taiwan's Defence Ministry says.

Key points: Liaoning aircraft carrier and five vessels enter South China Sea

Liaoning aircraft carrier and five vessels enter South China Sea 'Routine exercise' comes as tensions with Taiwan at a high

'Routine exercise' comes as tensions with Taiwan at a high China officially restores relations with Sao Tome and Principe

The self-ruled island said China had termed the move a routine exercise.

It comes amid renewed tension over Taiwan, which Beijing claims as its own, following US President-elect Donald Trump's telephone call with the island's President.

The Soviet-built Liaoning aircraft carrier has taken part in previous exercises, including some in the South China Sea, but China is years away from perfecting carrier operations similar to those the United States has practised for decades.

Taiwan's Defence Ministry said the carrier, accompanied by five vessels, passed southeast of the Pratas Islands, which are controlled by Taiwan, heading southwest.

The carrier group earlier passed 90 nautical miles south of Taiwan's southernmost point via the Bashi Channel, between Taiwan and the Philippines.

"Staying vigilant and flexible has always been the normal method of maintaining airspace security," said ministry spokesman Chen Chung-chi.

The ministry declined to say whether Taiwan fighter jets were scrambled or if submarines had been deployed.

'Signal to the US'

A member of Taiwan's opposition, Johnny Chiang, said the exercise was China's signal to the US that it has broken through the "first island chain", an area that includes Japan's Ryukyu Islands and Taiwan.

The US State Department on Monday said its position had not changed since July, when it said it was continuing to monitor China's military modernisation.

In Beijing, Chinese Foreign Ministry spokeswoman Hua Chunying said people should not read too much into what the carrier was up to, as its movements were within the law.

Sorry, this video has expired Satellite images show Subi Reef in the South China Sea before Chinese installations in 2012 and after in 2016.

China has been angered recently by US naval patrols near islands that China claims in the South China Sea.

Japan said late on Sunday it had spotted six Chinese naval vessels including the Liaoning travelling through the passage between Miyako and Okinawa and into the Pacific.

China claims most of the South China Sea through which about $7 trillion in ship-borne trade passes every year.

Sao Tome relations diplomatic victory for China

Meanwhile, China and Sao Tome and Principe officially resumed diplomatic relations in a triumph for Beijing over rival Taiwan after the African island nation abruptly broke away from the self-ruled island last week.

Chinese Foreign Minister Wang Yi and his counterpart from Sao Tome, Urbino Botelho, signed books at a ceremony in front of their flags at the Diaoyutai State Guesthouse in Beijing.

Mr Wang said the re-establishment of relations would bring benefits to both countries and they would hold exchanges in tourism, the media and other areas.

Most of the world does not formally recognise Taiwan as a condition of maintaining relations with China.

Sao Tome and Taiwan broke ties last week, leaving 21 countries and governments, mostly in Latin America and the Caribbean, that have official ties with Taiwan.

Reuters/ AP