Patriarch Bartholomew, the head of Istanbul's Fener Greek Orthodox Patriarchate, has been hospitalized in the United States capital Washington D.C.

Bartholomew, the spiritual leader of the world’s 200 million Eastern Orthodox Christians, arrived in the U.S. on Saturday for a 12-day visit that includes a meeting with President Joe Biden and has a broad agenda spanning religious, political and environmental issues.

The patriarch was due to attend a service at the Cathedral of Saint Sophia but felt unwell after the long flight and his busy schedule since reaching the U.S. capital, Greece-based AMNA news agency reported Sunday.

Bartholomew, 81, was taken to the George Washington University Hospital, where he was advised to rest and is being kept under observation, the press office of the Greek Orthodox Archdiocese of America said on Twitter.

Later Sunday, it said the patriarch "is feeling well” and was expected to be released Monday.

The patriarch is considered first among equals (primus inter pares) in the Eastern Orthodox hierarchy, which gives him prominence but not the power of a Catholic pope.

Making the latest of several trips to the country during his 30 years in office, Bartholomew is expected to address concerns ranging from a pending restructuring of the American church to his church’s status in his homeland, Turkey.

Bartholomew is scheduled to receive an honorary degree from the University of Notre Dame on Thursday in an event highlighting efforts to improve Orthodox-Catholic ties, centuries after the two churches broke decisively in 1054 amid disputes over theology and papal claims of supremacy.

Just as his influence is limited as Turkey does not recognize the patriarch as ecumenical, it is also limited in the Eastern Orthodox communion, rooted in Eastern Europe and the Middle East with a worldwide diaspora.

Large portions of the communion are in national churches that are independently governed, with the ecumenical patriarch having only symbolic prominence, though he does directly oversee Greek Orthodox and some other jurisdictions. The Russian Orthodox Church, with about 100 million adherents, has in particular asserted its independence and influence and rejected Bartholomew’s 2019 recognition of the independence of Orthodox churches in Ukraine, where Moscow’s patriarch still claims sovereignty.

In addition to his scheduled meetings with top U.S. officials, Bartholomew also plans to hold a ceremonial door-opening at St. Nicholas Greek Orthodox Church and National Shrine in New York City, which was built to replace a parish church destroyed during the 9/11 attacks, and to memorialize those killed at the nearby World Trade Center.

A 2017 Pew Research Center report found that there were about 200 million Eastern Orthodox worldwide. It reported about 1.8 million Orthodox in the United States, with nearly half of those Greek Orthodox.