World political and religious leaders expressed surprise but respect for Pope Benedict XVI's shock announcement Monday of his historic resignation due to "incapacity" because of his age.

France and Germany reacted within minutes of the news that the 85-year-old pontiff would step down as leader of the world's 1.1 billion Roman Catholics on February 28 because age was preventing him from carrying out his duties.

Describing the decision as "eminently respectable", French President Francois Hollande said his country, where the vast majority are of Catholic heritage, "hails the pope who took this decision."

German Chancellor Angela Merkel, herself the daughter of a pastor, said she had the "greatest respect" for the German-born pope's "difficult" decision.

"He is and remains one of the most significant religious thinkers of our time," Merkel added.

A spokesman for Merkel earlier said the pope deserved "gratitude" for his nearly eight years as pope, saying he had left a "very personal mark" on the Church, "both as a thinker and a shepherd".

"The federal government has the greatest possible respect for the Holy Father, for his accomplishments, for his life-long work for the Catholic Church," Steffen Seibert told a regular government news conference.

"Whatever the reasons may be for this declaration, they should be honoured and respected and he deserves gratitude for leading this world church for eight years in such a way," he added as news was breaking of the pope's resignation.

Pope Benedict was born Joseph Ratzinger in 1927 in the predominantly Catholic southern German region of Bavaria, whose state premier Horst Seehofer said the decision deserved the "greatest respect even though I personally deeply regret it".

Father Adam Boniecki, a close friend of Benedict's Polish predecessor John Paul II, commented that Benedict had taken note of the end of John Paul's papacy.

"I think that he did not want a repeat of those last dramatic months when the pope was still in his position but practically incapable of fulfilling his functions," he said.

The head of the Polish Catholic Church, Bishop Wojciech Polak, said Benedict's resignation was "a big surprise for us all".

"But Pope Benedict XVI had already reflected several times on the question of whether, at his advanced age, he had the strength to carry out properly his duties as the successor of Saint Peter," he said.

Prime Minister David Cameron of Britain, where Catholics are in a minority to Anglicans, said the pope "will be missed as a spiritual leader to millions".

"He has worked tirelessly to strengthen Britain's relations with the Holy See," Cameron said in a statement.

Israel's Chief Ashkenazi Rabbi Yona Metzger praised the pontiff for improving ties between Judaism and Christianity which helped reduced anti-Semitism around the world.

"During his term, the relations between the Chief Rabbinate and the Church, and Judaism and Christianity, became much closer, which brought to a decrease in anti-Semitic acts around the world," a spokesman for Metzger told AFP, expressing hope that his successor would continue in the same vein.

A spokesman for the foreign affairs department of the Russian Orthodox Church said he did not anticipate any major change of course as a result of the pope's resignation.

"There is no reason to expect radical changes in the Vatican's policy or its attitude towards orthodox churches," Russian agency Interfax quoted Dimitri Sizonenko as saying.

The pope has taken part in 24 official trips abroad since assuming the office in 2005, according to the Vatican website, as far afield as Mexico, Benin, Sydney and Brazil.

On his latest journey, a three-day visit to Lebanon in September, he condemned religious fundamentalism, called for peace in the Middle East and urged an end to supplies of arms to both sides in Syria's civil war.