Pope Francis is addressing the United Nations in a speech that is expected to focus on the environment and the poor.

His speech to the UN General Assembly comes a day after he became the first pontiff to address a joint meeting of Congress. 

In New York City, he will also visit ground zero, a school in East Harlem, go on a processional drive through Central Park and hold Mass for thousands at Madison Square Garden.

In his UN address, he warned world leaders of the dangers of climate change, a common theme of his public remarks in the US.

"The ecological crisis ... can threaten the very existence of the human species," he said.

He also railed against a litany of social evils from human trafficking to child sexual exploitation and terrorism.

He was also expected to tackle the topic of religious persecution.

Secretary General Ban Ki-moon greeted Francis on Friday as he became the fifth pontiff to visit the UN. 

The Vatican flag was raised for the first time just before his arrival, following the General Assembly's recent agreement to allow the Holy See's banner to be flown alongside those of 193 member states.

Later in the day, he will meet relatives of some of the nearly 3,000 victims of the 9/11 attacks before holding an interfaith service at the September 11 museum.

He will also visit Our Lady Queen of Angels School, set amid public housing in the Hispanic neighbourhood of East Harlem.

As many as 80,000 onlookers are expected to see him drive through Central Park, en route to a service for 18,000 at Madison Square Garden.

On Thursday evening, thousands cheered as Francis waved from his popemobile on Fifth Avenue on his way to St Patrick's Cathedral for evening prayers.

Francis wraps up his six-day US visit this weekend in Philadelphia, where he will speak in front of Independence Hall, among other engagements.