Pope Francis on Sunday became the first pontiff to set foot on the Arabian Peninsula, just hours after issuing his strongest condemnation yet of the war in Yemen, where his host the United Arab Emirates has a leading military role.

Shortly before departing for Abu Dhabi, Pope Francis said he was following the humanitarian crisis in Yemen with great concern, using his regular Sunday address in Vatican City to urge all sides to implement a fragile peace deal and help deliver aid to millions of hungry people.

'The cry of these children and their parents rises up to God,' he told tens of thousands of people in St Peter's Square.

'Let us pray strongly because they are children who are hungry, who are thirsty, they don't have medicine and they are in danger of death,' he said before boarding his flight.

The UAE welcomed the pope's message on Yemen and believes the peace deal he referred to was a historic breakthrough, Minister of State for Foreign Affairs Anwar Gargash wrote on Twitter.

'Let us assure its implementation and make 2019 the year of peace in Yemen,' he said after the pope landed in Abu Dhabi.

Yemen's warring parties agreed a ceasefire in December at the first major peace talks of the nearly four-year-old war, which pits Arab states backing an exiled president against the Iran-aligned Houthi movement that controls the capital.

Tens of thousands of people have been killed in the conflict, and the United Nations says millions are on the verge of starvation. Their survival depends on the truce preventing aid from being cut off by all-out fighting for the country's main port.

Francis was greeted by Abu Dhabi's Crown Prince Sheikh Mohammed bin Zayed al-Nahyan, who escorted him to meet Sheikh Ahmed al-Tayeb, the Grand Imam of Egypt's Al-Azhar mosque and university, one of the main seats of learning of Sunni Islam. The pontiff embraced him.

Both men will hold meetings with Pope Francis on Monday.

The UAE plays a leading role in the Saudi-led coalition battling the Houthis in Yemen. The United Nations is trying to implement a truce and troop withdrawal deal in the main Yemeni port of Hodeidah, agreed at the December talks as a step that could pave the way for negotiations to end the conflict.

Vatican officials have said it is not clear whether Pope Francis will address the sensitive subject in public or private during his visit to Abu Dhabi, which is aimed at promoting interfaith dialogue.

'New page'

The pope will meet Muslim leaders and celebrate an outdoor mass for some 135,000 Catholics, an event with no precedent on the Arabian peninsula. He has said the trip, which ends on Tuesday, is an opportunity to write 'a new page in the history of relations between religions'.

The UAE, which named 2019 its Year of Tolerance, says the visit reflects its history as a 'cradle of diversity'.

But it faces criticism from human rights groups for jailing activists, including Ahmed Mansoor, an Emirati who is serving a 10-year sentence for criticising the government on social media.

'We are calling on Pope Francis to raise the issue of their incarceration with his hosts, and urge their immediate and unconditional release,' Amnesty International said in a statement on Saturday. UAE authorities did not immediately reply to a request for comment.

The UAE is home to around 1 million expatriate Catholics, many from the Philippines. Another 1 million Catholics are believed to live in other countries in the Arabian Peninsula.

Priests and diplomats describe the UAE as one of the least restrictive environments in the Gulf for Christian worship. But like its neighbours, the UAE does not allow dissent or criticism of its leadership.

All Gulf states except Saudi Arabia allow Christian worship in churches or church compounds and other premises with special licences. In Saudi Arabia, non-Muslims pray in secret gatherings in private homes and embassies.

Abu Dhabi's crown prince is an ally of Saudi Arabia's Crown Prince Mohammed bin Salman, who has reached out to Christian representatives abroad as part of attempts to open up the conservative kingdom. Mohammed bin Salman's international reputation has eroded since the murder of journalist Jamal Khashoggi by Saudi agents inside the Saudi consulate in Istanbul in October.

The pope has already visited half a dozen predominantly Muslim nations during his reign, using the trips to call for inter-religious dialogue and to condemn the use of violence in the name of God.

The UAE ambassador to Washington, Yousef Al Otaiba, wrote in an op-ed in Politico magazine on Sunday that the pontiff's visit sent a signal of co-existence and respect in the face of the 'menace of extremism' across the Middle East.