Pope Francis has opened his visit to the United States with a call for action to combat climate change.

Speaking in front of a crowd of thousands at the White House, the pope called climate change a problem that "can no longer be left to a future generation".

US President Barack Obama hailed the pontiff as a moral force who is "shaking us out of our complacency" with reminders to care for the poor and the planet.

Pope Francis delivered a strong message against those who doubt the science of climate change, saying that the warming planet "demands on our part a serious and responsible recognition" of conditions awaiting today's children.

The Pope's message also had something for conservatives, with a call to protect religious liberties - "one of America's most precious possessions".

"All are called to be vigilant,' he said, "to preserve and defend that freedom from everything that would threaten or compromise it."

The message is expected to be welcomed by many US bishops and conservatives who have objected to the Obama administration's healthcare mandate and the recent Supreme Court legalisation of same-sex marriage.

The Pope and President stood on a red-carpeted platform decorated with red, white and blue bunting, standing at attention for the national anthems of the Holy See and the United States.

Just before the Pope arrived, Obama had tweeted: "Welcome to the White House, @Pontifex! Your messages of love, hope, and peace have inspired us all."

Mr Obama told the Pope that the excitement surrounding his visit was a reflection of Pope Francis' unique qualities, mentioning "your humility, your embrace of simplicity, the gentleness of your words and the generosity of your spirit".

The President singled out the Pope's call for focusing on the poor and the marginalised, including refugees fleeing war and immigrants in search of a better life. He also thanked the pope for his support for efforts to normalise relations between the US and Cuba

The 78-year-old pontiff is due to meet with America's 450-strong bishops' conference at the Cathedral of St Matthew the Apostle.

Many US bishops have struggled to come to terms with Pope Francis' new social justice-minded direction of the church. Nearly all were appointed by Popes John Paul II and Benedict XVI. They prioritised drawing clearer boundaries for Catholic behaviour and belief in the face of legalised abortion and advances in gay rights.

The American church spends hundreds of millions of dollars each year through its social service agencies, and for years has sought an overhaul of the immigration system to reunite families, shelter refugees and give the poor the chance at a better life.

But the US Conference of Catholic Bishops has put its resources behind high-profile fights over abortion, contraception and gay marriage.