The Bild daily, which first reported on the letters Thursday, gave more details of November correspondence it said it obtained between the German-born Benedict and Cardinal Walter Brandmueller - a prominent critic of Benedict's successor, Pope Francis. Many conservative critics long for Benedict's more doctrinaire papacy and question his decision to resign.
Brandmueller had said the previous month that Benedict's new title "does not exist in the entire history of the church" and he initially thought the resignation was a "carnival joke."
In a letter that Bild published a photo of, Benedict said he had "tried to create situation in which I am absolutely inaccessible for the media and in which it is completely clear that there is only one pope."
He also said that there had been previous, if very rare, papal resignations and reverting to being a cardinal wouldn't have made sense. He said that "whether intentionally or unintentionally, this could have led to difficult consequences in the context of the current situation."
Benedict's decision to take the title "emeritus pope," and continue to wear the white cassock of the papacy and live in the Vatican sparked controversy from the moment it was announced. While he has largely kept to his promise to retreat to a lifetime of prayer and meditation, he remains a point of reference for conservative and traditionalist Catholics alarmed by Francis' opening to allowing divorced and civilly remarried Catholics to receive Communion.
In a second letter to Brandmueller, whose reply Bild hasn't released, Benedict said he could understand the "deepseated pain" the end of his papacy caused.
"However, for some people and - it seems to me - also for you, the pain has turned into an anger that no longer merely concerns my resignation, but increasingly also my person and my papacy as a whole," he wrote. "By this, a papacy itself is now being devalued and melted into the sorrow about the situation in which the church currently finds itself."
That is leading to "a new kind of agitation," Benedict added. The letter didn't elaborate on the situation of the church.
The Vatican declined to confirm or deny the authenticity of the letters, or comment on them.