The Vatican on Monday denounced the self-ordination of a bishop from China's underground church without papal approval, in a sign some view as an attempt to improve ties with Beijing.

The Holy See and Beijing have not had diplomatic relations since 1951.

Millions of Chinese Catholics are outside the fold of the Communist Party's Chinese Catholic Patriotic Association. The Vatican has named some bishops from the group but they are not recognised by Beijing.

Catholic media sites said several priests from the underground church had named themselves bishop without the Vatican's blessings, including Paul Dong Guanhu, who also declared he could ordain others.

According to a news agency specialising in Asian churches, up to 10 such ordinations could have happened in the past years.

"In recent weeks, there has been a series of reports regarding some episcopal ordinations conferred without papal mandate of priests of the unofficial community of the Catholic Church in Continental China," the director of the Vatican's press office, Greg Burke, said.

"The Holy See has not authorised any ordination, nor has it been officially informed of such events," it said.

"Should such episcopal ordinations have occurred, they would constitute a grave violation of canonical norms," Burke added.

He said it was not "licit to proceed with any episcopal ordination without the necessary papal mandate, even by appealing to particular personal beliefs."

Since his election in early 2013, Pope Francis has sought to mend fences with China and informed sources say a pact in the

A Chinese delegation is expected in Rome in November "for a last round of negotiations," Belgian priest Jeroom Heyndrickx, who is an expert on China's Catholic Church and has close links with the Vatican, told AFP recently.

According to the terms of the agreement being drawn up, the pope will recognise four of the eight bishops it has refused to consecrate.

But it will sidestep the issue of some 30 bishops named by the Holy See who are not recognised by China. The planned accord, even if agreed, is not expected to lead to a resumption of diplomatic ties.