Pope Francis on Wednesday urged the United States 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 Argentine pontiff lauded President Barack 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. When it comes to the care of our common home, we are living at a critical moment of history," the pope said at a welcoming ceremony for his first visit to the United States.

The pope also invoked America'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 Obama greet the 78-year-old 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, Francis this year released a papal document, or encyclical, demanding swift action on climate change.

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," Obama said.

Francis and Obama were to hold talks in the White House after the ceremony. Both men see eye-to-eye on issues like climate change and defense of the poor but hold different views on abortion rights and gay marriage.

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 marginalized 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.

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."

After the White House meeting, Francis will parade through downtown Washington before a crowd expected to reach tens of thousands.