As Benedict XVI celebrates his 85th birthday and his seventh anniversary as pope this week, the leader of the Catholic Church has asked, amid concerns over his health, for prayers and the strength to carry on.

"Next Thursday, on the occasion of the seventh anniversary of my election to the See of Peter, I ask for your prayers, so that the Lord gives me the strength to fulfill the mission he entrusted to me," the pontiff said during the traditional Sunday blessing at St Peter's Square, in a report by the Huffington Post.

The pontiff is the oldest pope since Leo XIII, who died at the age of 93, in 1903, after reigning for 25 years.

Benedict was elected as the successor to Pope John Paul II in 2005 on the second day of the papal conclave after four ballots. He has, over the years, acquired a reputation as theologically conservative and his teachings and writings have defended traditional Catholic doctrine and values.

His declining health has raised speculation over whether he will be the first pope in 700 years to abdicate. Benedict had earlier said he would willingly resign once he "is no longer physically, psychologically, and spiritually capable of handling the duties of his office".

"I'm old but I can still carry out my duties," the pope told Fidel Castro during their meeting in Havana on his trip to Cuba last month.

Benedict had a tiring start this year with official visits to Mexico and Cuba. Returning from the tour, he was then seen celebrating Palm Sunday Mass at St Peter's Square, the first celebration in the busiest week of the liturgical year.

The music-loving pontiff was treated to a concert at his 80th birthday celebrations, when American violinist Hilary Hahn and Venezuelan conductor Gustavo Dudamel, who was recently appointed music director of the Los Angeles Philharmonic, came together for a performance. A similar celebration is expected for his milestone 85th birthday.

The pope will also be celebrating his birthday with his older brother, Monsignor Georg Ratzinger, who flew in from Germany over the weekend.