Top clerical leaders and heads of state reacted on Monday reacted to the resignation of Pope Benedict XVI, with many saying they respected the decision by the Roman Catholic leader to retire due to his advanced age.

They also expressed their appreciation for his work.

Pope Benedict, 85, issued a statement to a gathering of senior Catholic officials where he cited the "deterioration" of his mental and physical strength in recent months as reasons, saying he was no longer able to carry out his duties.

The Pope's resignation is the first by a sitting Pontiff in more than 600 years.

Senior Catholic officials are expected to convene in March during a "conclave" of Cardinals to elect a successor.

A leader of a global church body dedicated to Christian unity said the Pontiff's decision deserved respect.

"We have to respect fully the decision of His Holiness Pope Benedict XVI to resign," said Rev. Dr. Olav Fykse Tveit, secretary general of the World Council of Churches. "With deep respect I have seen how he has carried the responsibility and burdens of his ministry in his advanced age, in a very demanding time for the church."

Rev. Tveit said he appreciated the Pope's "love and commitment to the church and the ecumenical movement," noting Pope Benedict's membership in the organization's Faith and Order Commission in the 1960s and 1970s when he was a professor in Germany.

United States President Barack Obama said he and First Lady Michelle Obama "warmly" remembered the Pope's visit to the country in 2009.

"I have appreciated our work together over these last four years," Obama said.

He said the church "plays a critical role in the United States and the world, and I wish the best to those who will soon gather" to choose a successor.

United Kingdom Prime Minister David Cameron, speaking from the floor of Parliament extended his "best wishes" to Pope Benedict following the annoucement.

"He has worked tirelessly to improve Britain's relationship with the Holy See and his visit to Britain in 2010 is remembered with great respect and affection," Cameron said. "Pope Benedicts' message on that visit of working for the common good is something that spoke to our whole country and I'm sure his successor will continue to provide a voice of inspiration for millions around the world."

German Chancellor Angela Merkel said she respected the pope's decision.

"If the pope himself, after thorough reflection, has come to the conclusion that he doesn't have the strength anymore to carry out his duties, then this has my utmost respect," Merkel said in a brief statement issued in Berlin, according to Reuters. "He had to make a difficult decision." French Presiden Francois Hollande said the decision was "eminently respectable."

Leaders representing a fellowship of Lutherans around the world expressed appreciation for Pope Benedict's commitment ot the search for the unity of the church and his support to ongoing ecumenical dialogues.

The president of the Lutheran World Federation Bishop Dr. Munib A. Younan and LWF General Secretary Rev. Martin Junge said in a joint statement thta they offered their prayers for the pope and the Catholic church at this time.

They noted 50 years of dialogue between the LWF and the VAtican as an enriching mutual journey which Pope Benedict strongly supportted,

They quoted Pope Benedict saying durin a meeting with LWF leaders in the Vatican in 2010 that the dialogue had expressed gratitude for "the many significant fruits produced" by the bilateral discussions between Lutherans and Roman Catholics.

Cardinal Timothy Dolan attempted to encapsulate Pope Benedict's tenure, saying the pontiff reached out to people of various religions, the poor, and the youth around the world, unified Catholics, encouraged respect for "God's creation in nature" and for urged "people everywhere to know and have a personal encounter with Jesus Christ.

"We are sad that he will be resigning but grateful for his eight years of selfless leadership as successor of St. Peter," Cardinal Dolan said.

The United Nations in New York said in a statement that Secretary-General Ban Ki-moon recalled the Pope's visit to the U.N. headquarters in April 2008.

Ban expressed his "appreciation for the profound commitment of Pope Benedict to inter-faith dialogue and to addressing such global challenges as the alleviation of poverty and hunger, as well as the promotion of human rights and peace."