Pope Francis has met the head of the Russian Orthodox Church, Patriarch Kirill, in an historic development in the 1,000-year-old schism that divided Christianity.

Pope Francis and Patriarch Kirill embraced and kissed one another three times on the cheek as they met in a wood-panelled VIP room at Havana airport.

It was the first time a pope and Russian patriarch had ever met.

The Vatican sees the meeting as an important new step in its ecumenical efforts, but many Orthodox observers see Kirill's willingness to sit down with a pope as more an attempt to assert Russia and Russian Orthodoxy at a time when Moscow is being isolated by the West.

"Finally!" Pope Francis exclaimed as he embraced Patriarch Kirill, and Patriarch Kirill told the pope through an interpreter: "Now things are easier."

Pope Francis was having the brief talks in Cuba before heading off on a five-day visit to Mexico.

The meeting and signing of a joint declaration was decades in the making and cemented Pope Francis' reputation as a risk-taking statesman who values dialogue, bridge-building and rapprochement at almost any cost.

Indeed, while the meeting with Patriarch Kirill has been hailed by many as an important ecumenical breakthrough, Pope Francis has also come under criticism for essentially allowing himself to be used by a Russia eager to assert itself among Orthodox Christians and on the world stage at a time when the country is increasingly isolated from the West.

Pope Francis and Patriarch Kirill signed a joint declaration on religious unity after their historic meeting.

The declaration calls for peace in Syria, Iraq and Ukraine and urges Europe to "maintain its faithfulness to its Christian roots".

Pope Francis said: "We spoke clearly and directly. I greatly appreciate his desire for unity."

They also exchanged gifts that are deeply symbolic and trace the history of the sometimes tense relationship between East and West.

Pope Francis gave Patriarch Kirill a reliquary containing a relic of St Cyril, the 5th century archbishop of Alexandria who is revered by both Catholic and Orthodox churches. He also gave him a chalice.

Patriarch Kirill, for his part, offered Pope Francis a small replica of the Madonna of Kazan icon.

In 2004, the Vatican had returned an 18th century copy of the image to Patriarch Kirill's predecessor, Alexy II, in a bid to forge better ecumenical friendship.

The traditional Byzantine gold-and-wood icon depicts the Madonna and Child.

The original 16th century work was revered by Russian believers for its purported ability to work miracles, including the rout of Polish invaders in the early 17th century.

St John Paul II had hung it in his private chapel after receiving it from a Catholic group in 1993. He had hoped that returning an icon so revered by him personally might forge better ties with the Russian church.

While welcoming the return, Alexy said since it was only a copy of the original 16th century icon, the pope didn't need to personally accompany it back to Moscow, thus dashing his hopes for a visit.

The Vatican is hoping the meeting will improve relations with other Orthodox churches and spur progress in dialogue over theological differences that have divided East from West ever since the Great Schism of 1054 split Christianity.

But Orthodox observers say Patriarch Kirill's willingness to finally meet with a pope has less to do with any new ecumenical impulse than grandstanding within the West and the Orthodox Church at a time when Russia is increasingly under fire from the West over its military actions in Syria and Ukraine.

Patriarch Kirill, a spiritual adviser to Russian president Vladimir Putin, leads the most powerful of the 14 independent Orthodox churches that will meet this summer in Greece in the first such pan-Orthodox synod in centuries.

Catholic and Orthodox split in the Great Schism of 1054 and have remained estranged over a host of issues, including the primacy of the pope and, more recently, Russian Orthodox accusations that the Catholic Church was poaching converts in former Soviet lands.

Those tensions have prevented previous popes from ever meeting with the Russian patriarch, even though the Vatican has long insisted that it was merely ministering to tiny Catholic communities.