A war of words between a Hong Kong bishop and the Vatican escalated Friday with the outspoken Cardinal Joseph Zen calling the Pope's warming ties to Communist China "evil".

Beijing and the Vatican severed diplomatic relations in 1951, and although ties have improved in recent years as China's Catholic population has grown, they remain at odds over which side has the authority to ordain bishops.

The Vatican relaunched long-stalled negotiations three years ago and now seems to be nearing concrete steps towards solving the major stumbling block of how to designate bishops.

The issue has flared up again after two underground Chinese bishops, recognized by the pope, were asked by a top Vatican diplomat to resign in favor of state-sanctioned prelates, including one who was excommunicated by the Vatican in 2011.

Cardinal Joseph Zen, bishop emeritus of semi-autonomous Hong Kong and a staunch opponent of any rapprochement between the Vatican and Beijing, last month accused the Holy See of "selling out" to Beijing by reportedly promoting bishops endorsed by the Chinese government.

On Friday, the cardinal stepped up his rhetoric again.

"I am not objecting to an imperfect agreement -- that I can accept -- but not an evil one, not one that runs counter to the principle of our church," Zen said on a local radio program.

"They say they are doing it step by step, and yes it's fine to do it step by step, but you can't make a sinful step," he added.

China's roughly 12 million Catholics are divided between a government-run association, whose clergy are chosen by the Communist Party, and an unofficial church which swears allegiance to the Vatican.

The secretive negotiations between the two sides could come down to the Vatican recognizing some of the bishops chosen by the Chinese Patriotic Catholic Association in exchange for a more benevolent attitude from Beijing.

Zen revealed earlier that he had appealed to the pope in a private meeting in Rome last month, where he delivered a letter from one of the bishops who was asked to step aside, Zhuang Jianjian.

The Chinese Ministry of Foreign Affairs did not immediately respond to requests for comment.