Pope Francis on Thursday urged the US Congress to help those fleeing war and poverty, and work to combat climate change, in a historic address to lawmakers that touched on many hot-button issues in US and global politics.

The historic speech to lawmakers at the Capitol was keenly anticipated and warmly received -- but will have been uncomfortable listening for some there who are skeptical about environmental causes and opposed to immigration.

"Our world is facing a refugee crisis of a magnitude not seen since the Second World War," he warned, referring to the wave of refugees arriving in Europe from Syria, Iraq and Afghanistan.

"On this continent, too, thousands of persons are led to travel north in search of a better life for themselves and for their loved ones, in search of greater opportunities," he added, referring to immigrants from Mexico and Central America.

"Is this not what we want for our own children?" implored the 78-year-old Argentine pontiff, himself the son of European immigrants to the Americas.

"We must not be taken aback by their numbers, but rather view them as persons, seeing their faces and listening to their stories, trying to respond as best we can to their situation."

Many conservatives in the US Senate and House of Representatives deny that climate change is a result of mankind's industry and agriculture, and have opposed emissions controls designed to slow global warming.

The pope urged them to instead seek "courageous" and "responsible" solutions "to avert the most serious effects of the environmental deterioration caused by human activity."

"I am convinced that we can make a difference and I have no doubt that the United States -- and this Congress -- have an important role to play," he declared.

Underpinning the pope's message on specific policy areas was his concern about the danger posed by religious and political extremists.

"We know that no religion is immune from forms of individual delusion or ideological extremism. This means that we must be especially attentive to every type of fundamentalism, whether religious or of any other kind," he continued.

"A delicate balance is required to combat violence perpetrated in the name of a religion, an ideology or an economic system, while also safeguarding religious freedom, intellectual freedom and individual freedoms."