President Barack Obama called on wealthier nations Tuesday to do more to help millions of refugees find new homes and asked all leaders to imagine what it would be like "if the unspeakable happened to us."

In his final speech to the U.N. General Assembly, Obama offered praised for nations "right now that are doing the right thing" to help ease the largest refugee crisis since World War II. But he said many countries, "particularly those blessed with wealth and the benefits of geography," can do more to offer assistance to more than 65 million people who have fled their homes because of war or persecution or to seek a better life.

Obama commented as the White House announced that more than four dozen U.S. businesses have pledged $650 million to help refugees. Obama will also hold a special refugee summit later Tuesday on the sidelines of the annual gathering of world leaders.

Fifty-one corporations, including Facebook, Twitter, MasterCard, Johnson & Johnson, yogurt maker Chobani and others, have committed to easing access to education, employment and financial services for some 6.3 million refugees in more than 20 countries.

Countries participating in Obama's summit are announcing individual pledges in line with a U.S. goal of increasing humanitarian aid by $3 billion, doubling resettlement and providing access to jobs and education.

Obama welcomed the pledges of increased assistance.

"In the eyes of innocent men and women and children who through no fault of their own have had to flee everything that they know, everything that they love, we have to have the empathy to see ourselves," Obama said in his address to world leaders. "We have to imagine what it would be like for our families, for our children, if the unspeakable happened to us."

He said the world will be more secure if nations are prepared to help those in need and urged countries to follow through on their pledges "even when the politics are hard."

The millions of refugees leaving war-torn Syria and other countries wracked by conflict have led to a backlash in some countries, including in the U.S., where Republican presidential nominee Donald Trump has suggested a moratorium for Muslim immigrants.

Last week, the White House announced that the U.S. would resettle 110,000 refugees in the coming year, a 30 per cent increase over the 85,000 allowed in this year.

The 85,000 figure included 10,000 Syrian refugees, a figure advocacy groups had criticized as inadequate given the wealth of the U.S. and the fact that other countries, such as Canada and Germany, were welcoming far greater numbers of fleeing Syrians.

The administration has yet to release a country-by-country breakdown of the 110,000 refugee figure.

Before the summit, Obama planned to meet with CEOs of the corporations that have pledged support for refugees.

The refugee issue, meanwhile, was for the first time the subject of a daylong U.N. summit on Monday. Leaders approved a declaration to provide a more co-ordinated and humane response to the crisis, but the document included no concrete commitments and is not legally binding.

Associated Press writer Michael Astor contributed to this report.