UNITED NATIONS — Facing an escalating nuclear threat from North Korea and the mass flight of minority Muslims from Myanmar, world leaders gather at the United Nations starting Monday to tackle these and other tough challenges — from the spread of terrorism to a warming planet.

The spotlight will be on U.S. President Donald Trump and France’s new leader, Emmanuel Macron, who will both be making their first appearance at the General Assembly. They will be joined by more than 100 heads of state and government, including Zimbabwe’s President Robert Mugabe, one of Africa’s longest-serving leaders who is said to be bringing a 70-member entourage.

While Trump’s speeches and meetings will be closely followed, it will be North Korea, which Secretary-General Antonio Guterres calls “the most dangerous crisis that we face today,” that will be most carefully watched. No official event addressing Pyongyang’s relentless campaign to develop nuclear weapons capable of hitting the United States is on the U.N. agenda, but it is expected to be the No. 1 issue for most leaders.

Not far behind will be the plight of Myanmar’s Rohingya Muslims, victims of what Guterres calls a campaign of ethnic cleansing that has driven nearly 400,000 to flee to Bangladesh in the past three weeks. The Security Council, in its first statement on Myanmar in nine years, condemned the violence and called for immediate steps to end it. British Foreign Secretary Boris Johnson is hosting a closed meeting on the crisis Monday, and the Organization of Islamic Cooperation’s contact group on the Rohingyas is scheduled to meet Tuesday.

Guterres said leaders would also be focusing on a third major threat — climate change. The number of natural disasters has nearly quadrupled and he pointed to unprecedented weather events in recent weeks from Texas, Florida and the Caribbean to Bangladesh, India, Nepal and Sierra Leone.

While Trump has announced that the United States will pull out of the 2015 Paris Climate Agreement, Macron will be hosting a meeting Tuesday to spur its implementation. And a late addition to the hundreds of official meetings and side events during the ministerial week is a high-level session Monday on the devastation caused by Hurricane Irma.

Several terrorism-related events are on the agenda. Macron is holding a meeting Monday with leaders of five African nations — Mali, Mauritania, Niger, Burkina Faso and Chad — that are putting together a 5,000-strong force to fight the growing threat from extremists in the vast Sahel region.

A side event Wednesday on “Preventing Terrorist Use of the Internet” will be attended by senior representatives of major social media companies. Co-hosts Britain, France and Italy said a global response is needed “to make the online space a hostile environment for terrorists.”

Trump has accused Iran of supporting terrorists and is threatening to rip up the 2015 deal to rein in its nuclear program. With a U.S. decision due in October, ministers from the six parties to the agreement are expected to meet next week. The five others strongly support the deal.

Trump has also been critical of the United Nations and has promised to cut the U.S. contribution to its budget, which is the largest. So some diplomats were surprised that the United States would sponsor an event Monday on reforming the 193-member world body.