The visit by Bishop Stephen Chow and several of his senior priests at the invitation of Bishop of Beijing Joseph Li Shan comes weeks after the Vatican announced that China had unilaterally installed a new bishop to Shanghai, an appointment that took the Vatican by surprise.

The Vatican said it was informed of the decision just a few days before the Chinese announcement, and has previously accused China of violating a bilateral accord, renewed last October, over the appointment of bishops.

The deal struck in 2018 was a bid to ease a longstanding divide among mainland China's roughly 12 million Catholics between an underground Church that swears loyalty to the Vatican and the state-supervised Catholic Patriotic Association.

For the first time since the 1950s, both sides recognised the pope as supreme leader of the Catholic Church.

In a statement to Reuters ahead of his five-day trip, Chow - who was appointed by the pope - said he hoped to "promote exchanges and interactions" between the mainland and the broader Church, in particular with Asia.

When asked if he was carrying any message for his hosts from the Vatican, Chow, through a spokesperson, said: "This is an exchange of visits between dioceses. Mainland China has established liaison with the Vatican on state affairs."

China's National Religious Affairs Administration and its state-backed Chinese Catholic Patriotic Association did not immediately respond to requests from Reuters for comment.

Vatican officials have said the 2018 deal, the details of which have not been disclosed, does not apply to Hong Kong given the "one country, two systems" arrangement that secured its handover from British to Chinese rule in 1997.

Freedom of religious belief and worship is detailed in Hong Kong's mini-constitution, known as the Basic Law, which has governed the city's status as a Special Administrative Region of China since Britain handed it over in 1997.

Some Hong Kong priests and Catholics fear Chinese authorities are gradually tightening control over the church in the city of just over 7 million people, about 400,000 of whom are Catholic, in part through the growth of exchanges between senior clerics.

Chow did not rule out raising those concerns with his mainland counterparts.

"It is possible to touch on different religious or other concerns. Up to now, there is no restriction in religious freedom in Hong Kong per se," he said.

The Hong Kong government did not immediately respond to a request for comment.

Pope Francis named Chow as bishop of Hong Kong in May 2021 - a long-delayed appointment amid growing Western concern over human rights and freedoms in the Asian financial hub.