Ties between the Vatican and Beijing have been fraught further with the Holy See excommunicating a Chinese bishop and threats by the China's state-run Church to continue defying Pope Benedict XVI.

China's 5.7 million Catholics are increasingly caught between showing allegiance to the Patriotic Catholic Association or to the pope as part of an "underground" Church not recognised by the authorities.

The Vatican on Monday confirmed the excommunication of Paul Lei Shiyin, who it said had been ordained "illegitimately" last week in Sichuan province in the presence of seven bishops who are recognised by the Holy See.

Lei Shiyin "has no authority to govern the diocesan Catholic community, and the Holy See does not recognise him," a statement said.

The seven bishops had "exposed themselves to serious canonical sanctions" that could even lead to excommunication, though Vatican spokesman Federico Lombardi suggested to AFP there may have been mitigating circumstances.

The Holy See said "an episcopal ordination without Papal mandate is directly opposed to the spiritual role of the Supreme Pontiff and damages the unity of the Church," as well as "deeply saddening" Pope Benedict XVI.

"If the church in China wants to be Catholic, it must respect the doctrine and discipline of the Church," it said.

Unlike the modus vivendi the Vatican has achieved with other communist countries, the pope and China have never found common ground.

"This excommunication shows up great disenchantment" on Benedict XVI's part, Vatican specialist Marco Politi said, adding that the pope was doubly frustrated after having "reached out" to the regime in 2007 to try and improve ties.

The Holy See had promised bishops from China's official Church that they would be recognised if they accepted the pope's supremacy.

While some took the plunge, Vatican experts believe others continue to be attracted by the advantages offered by the official Church.

One of the difficulties the Vatican faces is the regime's "constantly fluctuating" attitude, Politi said.

The Vatican had hoped China would guarantee religious freedom while opening up its economy. Beijing had initially agreed to postpone new bishop ordinations but its attitude hardened again in 2010.

Tensions arose after a bishop was ordained without permission in Chengde in northern China last November.

In December, the Vatican complained that the National Assembly of Catholic Representatives had "imposed" itself on "numerous bishops, priests, religious and faithful people."

The pope then launched an anguished appeal on May 18 calling on all bishops to "refuse to take the path of separation," in spite of "pressure" from the communist authorities.

But the Patriotic Catholic Association went announced at the end June that it hoped to ordinate 40 bishops, adding that it was "the best way to spread the gospel in China."

Despite the alligiance of many Catholics to the Vatican, the Holy See has watched worriedly as the state-run Church appears to enjoy great success.

Savio Hon Tai Fai, the Chinese deputy head of the Congregation of the Evangelization of Peoples, said it showed a "certain weakness inside the Chinese Church.

"An ambitious priest, one keen to make a career for himself, can be well looked upon by the government," he said.