Pope Francis has begun a three-day visit to Bulgaria and North Macedonia aimed partly at improving relations with the Orthodox Church.
The Pope met Orthodox leaders in Bulgaria on Sunday, but they had already rejected taking part in joint services or prayers with him.
The Vatican has long been pushing for unity between the two branches of Christianity, which split in 1054.
The two Balkan countries have tiny Catholic communities.
The pontiff's arrival in Bulgaria on Sunday marked the first visit by a pope to the country in 17 years.
The trip to North Macedonia will be the first by a pope and comes just months after the country ended a decades-long dispute with Greece by changing its name, and as it votes for a new president.
Healing the schism
Central to the Balkan trip is the Vatican's desire to heal the 1,000-year-old schism that has divided Christianity.
The Pope met Orthodox Patriarch Neophyte and visited an Orthodox cathedral in the capital, Sofia, on Sunday.
But the Bulgarian Orthodox Church rejected the idea of joining prayers or services with him.
In a statement last month, it explained its position, saying that the invitation for the Pope's visit was made by state authorities.
In what is being viewed as an effort not to upset other Orthodox churches, the Pope is not expected to meet privately with Macedonian Orthodox Primate Stephen in his upcoming visit.
The Macedonian Orthodox Church declared its independence, or autocephaly, in 1967 but has not been recognised by other Orthodox churches.
Aside from moves to heal the rift with the Orthodox Church, the pontiff will meet the Balkan countries' minority Catholic communities.
Sister Elka Staneva, a nun in Rakovski, Bulgaria's largest predominantly Roman Catholic town, described the excitement there ahead of the visit.
"It is a great joy, a great spiritual experience, a feast of faith for the whole community here in Rakovski as well as for the whole country," she told Reuters.
There are an estimated 58,000 Catholics in Bulgaria and 15,000 in North Macedonia.
Speaking at the presidential palace in Sofia on Sunday, Pope Francis addressed the issue of low birth rates and high levels of emigration in Bulgaria.
"Bulgaria, like so many other countries of Europe, must deal with what can only be called a new winter: the demographic winter that has descended like an ice curtain on a large part of Europe, the consequence of a diminished confidence in the future," he said.
The Pope called on Bulgaria to "strive to create conditions that lead young people to invest their youthful energies and plan their future, as individuals and families, knowing that in their homeland they can have the possibility of leading a dignified life."
He also urged government officials "not to close your eyes, your hearts or your hands" to migrants.
Bulgaria is the European Union's poorest member state, and has seen some two million people leave the country since the fall of communism in search of new opportunities and better living standards.