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 Donald 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.

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."

To understand the comment, 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."