UN member-states adopted a plan on Monday to tackle the biggest refugee crisis since World War II amid dire warnings about rising xenophobia in Europe and the collapse of countries bordering war-shattered Syria.

The first-ever UN summit on refugees and migrants kicked off a week of high-level diplomacy as world leaders take part in the annual General Assembly meeting, dominated this year by the conflict in Syria.

Speaking at the summit opening, UN human rights chief Zeid Ra'ad Al Hussein made a rousing appeal to confront "race-baiting bigots," recalling that the refugee crisis stemmed from the world's failure to end the war in Syria.

"This should not be a comfortable summit," he told the gathering at the UN General Assembly.

"The defenders of what is right and good are being outflanked in too many countries by race-baiting bigots, who seek to gain, or retain, power by wielding prejudice and deceit at the expense of those most vulnerable," he said.

World governments adopted a non-binding political declaration pledging to uphold the rights of refugees, help them resettle and ensure they have access to education and jobs.

A record-breaking 65 million people are on the move worldwide, fleeing wars such as the carnage in Syria, repression and poverty, including 21 million refugees competing for too few resettlement opportunities.

Now in its sixth year, the war in Syria has driven nearly nine million people from their homes while an additional four million have fled to neighboring countries or are making the perilous journey to Europe.

A week-long fragile ceasefire deal brokered by Russia and the United States collapsed when the Syrian army announced the end of the truce and re-started air raids on rebel-held Aleppo.

- Facing collapse -

Lebanon's Prime Minister Tammam Salam, whose country of four million has taken in 1.5 million Syrians, appealed for urgent help.

"Barring a massive effort from the international community, Lebanon runs the risk of a serious collapse," he said. "My country is in serious danger."

Greek Prime Minister Alexis Tsipras said the stakes were also high for his country, which has seen more than 1.2 million migrants cross its border over the past year.

A failure to confront the refugee crisis will unleash xenophobia, he warned.

"If we fail to support this, the political repercussions will be felt not only in Greece but everywhere," he said.

"We will give space to nationalistic, xenophobic forces to show their face for the first time since the Second World War."

Turkey, home to 2.7 million Syrians, said no country can afford to turn its back on the refugee crisis.

"If there is one lesson that the entire world must have learned from now, no one is immune from the consequences of the refugee crisis today," Turkish Foreign Minister Mevlut Cavusoglu said.

Western Europe "must be more cautious than ever" in preventing Islamophobia, he added.

UN Secretary-General Ban Ki-moon launched a global campaign against xenophobia, saying it would seek to "turn fear into hope" at a time when welcoming migrants and refugees has become a divisive issue.

US President Barack Obama will host a second summit on Tuesday, when some 40 countries are expected make new offers of aid, either by taking in more refugees or supporting access to education and jobs.

China pledged $100 million in humanitarian aid for refugees on Monday, saying a $1 billion fund it has set up with the United Nations could be used to help address the crisis.