Pope Francis has told US Catholic bishops that "crimes" of sexual abuse of minors by clergy should never be repeated.

"I know how much the pain of recent years has weighed upon you, and I have supported your generous commitment to bring healing to victims ... and to work to ensure that such crimes will never be repeated," he told the bishops on his first visit to the United States. 

During his six days in the US, the pope may meet privately with victims of sexual abuse.

The Vatican has said an eventual meeting would be announced after it takes place in order to protect the privacy of the victims.

An estimated 6,400 Catholic clergy have been accused of abusing minors in the United States between 1950 and 1980.

In June, Francis sacked two US bishops accused of looking the other way: the archbishop of Saint Paul and Minneapolis, John Clayton Nienstedt, and his aide Lee Anthony Piche. 

And earlier this month the Vatican replaced Bishop Robert Finn of Kansas City, who resigned in April after failing to report a priest accused of pedophilia. 

Earlier, cheering crowds greeted Pope Francis in the streets of Washington as he rode in his open pope mobile near the White House following talks with President Barack Obama.

The 78-year-old Argentine pontiff, flanked by security guards, waved to spectators and pointed to specific people in the crowd as he slowly made his way down Constitution Avenue.

Pope Francis urged the US to help tackle climate change at a" critical moment of history" and called on Americans to build a truly tolerant and inclusive society.

In a speech on the White House South Lawn, the pontiff lauded President Obama's efforts to reduce air pollution.

"It seems clear to me also that climate change is a problem which can no longer be left to a future generation.

In Pictures: Pope Francis in Washington DC

When it comes to the care of our common home, we are living at critical moment of history," the pope said at a welcoming ceremony for his first visit to the United States.

The pope also invoked the US's best known civil rights leader, the late Rev Martin Luther King, to make his point on the environment.

About 15,000 people who gathered in bright sunshine on the South Lawn watched President Obama greet the pope, who departed from his usual practice and addressed the crowd in English.

A frequent critic of the damage caused to the world's poor and the environment by capitalism's excesses, Pope Francis this year released a papal document, or encyclical, demanding swift action on climate change.

President Obama, whose plans for a climate change bill were thwarted in Congress early in his presidency, said he shared the pope’s concerns about the environment.

"Holy Father, you remind us that we have a sacred obligation to protect our planet - God's magnificent gift to us.

We support your call to all world leaders to support the communities most vulnerable to a changing climate and to come together to preserve our precious world for future generations," President Obama said.

The pope paraphrased King's 1963 "I Have A Dream" speech, saying that the world has "defaulted on a promissory note" to the planet and millions of marginalised people.

"American Catholics are committed to building a society which is truly tolerant and inclusive, to safeguarding the rights of individuals and communities, and to rejecting every form of unjust discrimination," he said.

Pope Francis gave his support to traditional marriage in his remarks, pointing out that he will travel to Philadelphia later in his six-day visit to the United States for a meeting of Catholics "to celebrate and support the institutions of marriage and the family." 

Pope canonises controversial 18th-century missionary

Pope Francis has canonised an 18th-century missionary known by admirers as the "Apostle of California" but accused by Native Americans of helping to eradicate their culture.

The man now known as Saint Junipero Serra arrived in what is now San Diego in 1769 and went on to found nine of the 21 missions that grew into modern-day California.

A crowd of some 25,000 people including some opponents of the canonisation flocked around the Basilica of the National Shrine of the Immaculate Conception, where Francis said an outdoor mass and declared Serra a saint.

The ceremony also was the first canonisation carried out on US soil.

Detractors contend that Serra essentially imprisoned Native Americans in closed communities, where he suppressed their cultures and had them beaten as he tried to indoctrinate them in Roman Catholic ways.

Supporters acknowledge that corporal punishment was used but contend that was common practice at the time.

Francis said on a summer visit to Latin America that "many grave sins" had been committed against Native American people in God's name but said today that Serra had stood up for the residents of his missions.

"Junipero sought to defend the dignity of the native community, to protect it from those who had mistreated and abused it," the pope said in his homily delivered in Spanish on the steps of the largest Catholic church in North America.

The pope spoke of "mistreatment and wrongs which today still trouble us, especially because of the hurt which they cause in the lives of many people."

But he also seemed to acknowledge some criticisms of Serra, saying, "Mission is never the fruit of a perfectly planned programme or a well-organised manual."