ISTANBUL (AFP) - Pope Francis on Sunday joined forces with Orthodox Ecumenical Patriarch Bartholomew I to make a rare joint pledge of support for the embattled Christians of the Middle East.

On the final day of his first visit to Turkey, Francis also urged an end to the millennium-old schism between the Orthodox and Catholic churches and said this was all the more urgent due to the violence against Christians by ISIS (Islamic State in Iraq and Syria) extremists.

The trip of the pope to Istanbul - once the capital of the Christian Byzantine world and formerly known as Constantinople - has been marked by his overtures to reach out both to Muslims and other Christian confessions.

He left aboard the papal plane in the early evening, also finding time to address around 100 refugees displaced by the violence in Iraq and Syria.

The pope early Sunday attended a divine liturgy led by Orthodox Ecumenical Patriarch Bartholomew I, the "first among equals" of an estimated 300 million Orthodox believers.

In a joint declaration, Bartholomew and the leader of the world's Roman Catholics pledged to support Christians in the Middle East, saying they could not let Christianity be driven out of the region.

"We cannot resign ourselves to a Middle East without Christians, who have professed the name of Jesus there for two thousand years," the church leaders said.

They said the "terrible situation" of Christians calls "for an appropriate response on the part of the international community".

The two church leaders also called on the parties involved in the Ukraine conflict "to pursue the path of dialogue and of respect for international law".

Pope Francis in an address at the Orthodox Patriarchate on the banks of the Golden Horn urged an end to the schism between Orthodox and Catholic churches to bring them back into full communion.

"The one thing that the Catholic Church desires, and that I seek as Bishop of Rome... is communion with the Orthodox churches," he said. "How can we credibly proclaim the message of peace which comes from Christ if there continues to be rivalry and disagreement between us?"

Bartholomew for his part said that while the road to full communion would be "perhaps lengthy and sometimes even rugged" it was irreversible. He echoed the pope's comments that the violence against Christians had made this more pressing, saying: "We no longer have the luxury of isolated action."

The pope and Bartholomew have in the last months worked hard for a rapprochement between the eastern and western churches which have been split since the schism of 1054.

The reconciliation began in 1964 with the famous embrace in Jerusalem between Pope Paul VI and Ecumenical Patriarch Athenagoras, the first such meeting since the 15th century.

Bartholomew, who commands considerable respect beyond the Orthodox Church, holds an office that dates back to the early days of the Byzantine Empire, over a millennium before the Ottoman conquest of Constantinople in 1453.

The patriarchate in Istanbul remains his headquarters and the patriarch himself must under Turkish law be a citizen of the country.

During a prayer service on Saturday, the pope bowed his head and asked Bartholomew to kiss him on his brow, in a remarkable sign of humility towards the patriarch.

In another hugely symbolic moment, the pope during a visit Saturday to Istanbul's Ottoman Sultan Ahmet mosque - better known abroad as the Blue Mosque - turned towards Mecca and stood in two minutes of reflection next to a top Islamic cleric.

The trip has been marked by crowds far thinner than on Francis's previous visits abroad but also the heaviest security, which extended to positioning snipers on the balconies of mosque minarets.

Turkey's own Christian community is tiny - just 80,000 in a country of some 75 million Muslims - and only a small proportion of these are Catholics.

The pope, making only the fourth papal visit to Turkey, has at times looked fatigued during a crammed three-day programme but was often seen breaking into a smile at the sight of an old acquaintance.

His trip has been less controversial than the last by a pontiff to mainly Muslim Turkey - the visit by Pope Francis' predecessor Benedict XVI in 2006 which was overshadowed by remarks he had previously made deemed to be anti-Islamic.

Vatican spokesman Federico Lombardi described the atmosphere this time as more "cordial and serene".