CASTEL GANDOLFO, Italy (AP) — Retired Pope Benedict XVI has spent his 86th birthday behind closed doors at the papal retreat south of Rome amid concerns for his health and prayers from his successor.

Pope Francis called Benedict to wish him well and offered up his Tuesday morning Mass in his honor, praying "May God be with him, comfort him and give him much solace."Benedict appeared frail and weak the last time he was seen in public when he greeted Francis at Castel Gandolfo on March 23, a remarkable decline from just three weeks earlier.

The Vatican has denied Benedict is suffering from any illness, insisting he is simply old and tired. Before he left the Vatican, Pope Benedict said that in retirement he would remain "hidden from the world," and he has kept his word.