A possible candidate in Taiwan's presidential elections met the man in charge of handling the Chinese mainland's affairs with the self-ruled island on Friday in what observers say was a calculated but daring move to show off his credentials as someone Beijing can trust.

Parts of the meeting between Taipei Mayor Ko Wen-je, from the Kuomintang party, and Liu Jieyi, head of the Taiwan Affairs Office, which came at the end of Ko's three-day visit to Shanghai, were broadcast live via Facebook.

The meeting with Liu was an important part of Ko's trip, during which he also attended the 10th twin city forum between Taipei and Shanghai and presented himself as an alternative to the traditional mainland-friendly politicians from Taiwan's major opposition party, the Kuomintang, observers said.

Neither of the two men mentioned the "one China" principle that Beijing insists on as the political foundation of cross-strait relations.

But Liu said the two sides belonged to "one big family". Ko had also used the phrase to describe cross-strait ties and that might have helped him to get a green light from Beijing for his Shanghai visit.

Beijing, which considers Taiwan a wayward province awaiting reunification with the mainland " by force if necessary " has suspended official exchanges with Taiwan since Tsai Ing-wen of the independence-leaning Democratic Progressive Party became president in 2016 and refused to accept the one-China principle.

"The exchanges and cooperation between the compatriots on the two sides of the Taiwan Strait have become increasingly closer ... and to the extent that they are one big family," Liu said.

He said also that Chinese President Xi Jinping's message to Taiwanese compatriots, made in January, had "indicated the direction for the development of cross-strait ties".

Ko did not comment on Xi's message, but stressed the importance of cross-strait exchanges and mutual understanding.

Taiwan's Mainland Affairs Council said it was closely following the meeting. It had earlier warned Ko against meeting Liu without reporting to the island's authorities beforehand.

Known front-runners in the January presidential race have in recent months expressed hawkish views towards the mainland for fear that adopting a conciliatory posture would cost them voter support.

Taiwan has recently tightened rules on contact with the mainland as a result of strained relations between the two sides. To increase the pressure on Tsai, Beijing has staged a series of war games around Taiwan and poached three of its diplomatic allies.