The Vatican and China are preparing to renew a historic deal on the appointment of bishops that has slightly thawed icy relations -- much to the displeasure of the US.

Pope Francis has been working hard to repair ties with the Communist country, but his overtures run contrary to US President Donald Trump's efforts to push a religious freedom theme against China in his campaign for a second term.

The president's secretary of state Mike Pompeo went on the offensive last week in an article in US religious magazine First Things, slamming the "horrific" persecution of believers of all faiths in China.

He wrote that many countries had expressed "revulsion" at "accelerating violations of human rights".

"The Vatican endangers its moral authority, should it renew the deal", he added on Twitter.

China's roughly 12 million Catholics have for decades been split between a government-run association, whose clergy are chosen by the atheist Communist Party, and an unofficial underground church loyal to the Vatican.

China has around 12 million Catholics
China has around 12 million Catholics AFP / Johannes EISELE

The latter recognises the pope's authority and is often persecuted for it.

After years of snail-pace negotiations, the Vatican sealed a historical "provisional" agreement with Beijing on 22 September 2018, the exact content of which has never been published.

The key novelty however was that both Beijing and the Vatican now have a say in appointing Catholic bishops.

Francis immediately recognised eight Chinese bishops that had been appointed by Beijing without his approval.

Since then, two new bishops have been appointed in China, with the nod of the head of the world's 1.3 billion Catholics.

And in a historic step earlier this year, Chinese and Vatican foreign affairs ministers met publicly at an international event for the first time in seven decades.

China and the Vatican now have a say in appointing bishops
China and the Vatican now have a say in appointing bishops AFP / Nicolas ASFOURI

Pope Francis has just given the go-ahead for the renewal of the agreement, which is still in "experimental" mode, for another two years, a source close to the dossier told AFP.

The extension is expected to be signed next month.

Thorny issues, such as Chinese Catholic priests suddenly disappearing from their parishes for weeks "at the invitation" of the authorities, are in the meantime being raised with Beijing by Vatican diplomats, the source said.

One of the Catholic Church's leading experts on Chinese affairs, Father Bernardo Cervellera, told religious news site Cruxnow.com earlier this month that the deal may have drawn much fanfare but had so far borne "very little fruit".

And he voiced his hope that the Vatican, in renewing the agreement, would be tougher on China.

Diplomatic relations between Beijing and the Holy See broke down in 1951, two years after the communists came to power.

Efforts to rekindle them are hampered by the Vatican's decision to maintain diplomatic relations with Taiwan. The island, with a population of 23 million, is considered by Beijing to be a breakaway Chinese province awaiting reunification.

The Vatican is Taiwan's only diplomatic tie in Europe and Cervellera said he feared China will demand it break it off.

A spokesman for China's foreign ministry said Thursday that the interim deal with the Vatican has been "implemented successfully", saying there had been an increase in "mutual trust and consensus".

Pope Francis's right-hand man, Cardinal Pietro Parolin, said mid-September that the Catholic Church's "current interest with China is to normalise the life of the church as much as possible".

He admitted the results so far "have not been particularly striking".