UNITED NATIONS -- Secretary-General Antonio Guterres kicked off the United Nations General Assembly's annual debate on Tuesday by warning that we're living in "a world in pieces," and some very thinly veiled jabs at the current U.S. leader's efforts to solve the biggest problems on the planet.

"Our world is in trouble," Guterres said in his speech, which served as a prelude to speeches later Tuesday by world leaders including President Trump. "People are hurting and angry. They see insecurity rising, inequality growing, conflict spreading and climate changing."

The U.N. chief went on to outline seven key threats facing the world, and the major challenges to resolving them: the risk of nuclear conflict, international terrorism, unresolved conflicts and violations of international humanitarian law, climate change, rising inequality, cybersecurity, and the refugee crisis.

Get Breaking News Delivered to Your Inbox

In an unusual pre-debate event on Monday, Guterres, Mr. Trump and U.S. Ambassador Nikki Haley sat side by side, collaborating in support of reforming the United Nations.

At the U.S.-sponsored meeting, aimed at supporting Guterres' reform agenda, the U.N. chief thanked Haley for "her leadership, her partnership and her commitment," saying the global body's bureaucracy and endless red tape keep him up at night.

Mr. Trump's words were also unusually positive about the U.N., saying it has been at the forefront of "feeding the hungry, providing disaster relief, and empowering women and girls in many societies all across the world."

But a day after the display of unity by the U.N. and U.S. leaders, Guterres took what appeared to some diplomats to be a not-so-subtle jab at Mr. Trump and his efforts to curb immigration and refugee intake.

Without mentioning any country in particular, Guterres said Tuesday that, "instead of closed doors and open hostility, we need to reestablish the integrity of the refugee protection regime and the simple decency of human compassion.

"With a truly global sharing of responsibility, the number of refugees we face can be managed. But too many states have not risen to the moment," he said.

To understand the comments, one need only look at Guterres' background as former High Commissioner of the U.N. Refugee agency for a decade, making his name protecting migrants. As he opened the General Assembly's debate, Guterres spoke about the importance of refugees in a broader context.

"I myself am a migrant," Guterres said, "as are many of you… But no one expected me to risk my life on a leaky boat or cross a desert in the back of a truck to find employment outside my country of birth."

Dealing with the realities of the refugee crisis must be a global task, Guterres said, mentioning the thousands who have died trying to reach Europe in perilous sea crossings from Africa and the Middle East, and the still-developing crisis as Rohingya Muslims flee alleged persecution in Myanmar.

Climate change was a major focus of Guterres' speech as well, coming little over a month after the Secretary-General received notice of the Trump administration's intention to withdraw from the accord, which he called a "major disappointment." Since that time, Secretary of State Rex Tillerson has said there is some chance the U.S. could remain party to the agreement -- under the right conditions.

Guterres singled out North Korea, condemning the rogue state's nuclear and missile tests and saying the Security Council's unanimous adoption of sanctions sent a clear message: "Only that unity can lead to the denuclearization of the Korean Peninsula and -- as the resolution recognizes -- create an opportunity for diplomatic engagement to resolve the crisis."

Then, clearly admonishing both Kim Jong Un and Mr. Trump, Guterres warned that "fiery talk can lead to fatal misunderstandings."

In the end, Guterres returned to the theme that he, Mr. Trump and Haley discussed -- and agreed upon -- on day 1, that of reform: "We need to reform our world, and I am committed to reforming our United Nations."

Cultures, religions and traditions vary, at times compete, and at times, result in open conflict, Guterres said, "That is exactly why multilateralism is more important than ever."

The General Assembly debate began Tuesday and will continue through September 25, with 100 Heads of State and Government speaking to the 193-member world body.