United Nations (CNN) United Nations Secretary-General Antonio Guterres on Tuesday held his first news conference since taking office six months ago -- and had a message for President Donald Trump's administration.

Always careful with his phrasing , the leading UN diplomat didn't mention Trump by name -- but Guterres clearly warned his administration against isolating the United States from the rest of the world.

Creating a vacuum in international relations is risky, Guterres told a packed news conference room: "If the United States disengage in relation to many aspects of foreign policy and many aspects of international relations, it will be unavoidable that other actors will occupy that space," he said. "And I don't think this is good for the United States, and I don't think this is good for the world. "

Last month, Guterres publicly questioned Trump's decision to withdraw from the landmark Paris climate change accord in a rare public admonishment.

The UN leader is under considerable pressure by the US to reduce peacekeeping operations and slash spending. He also has to walk a careful line with the US, which is the largest contributor of funding to the UN, above 192 other nations.

Guterres conducted his first news conference on the occasion of World Refugee Day. In a 17-minute speech at the outset, Guterres said countries around the world are closing more borders and turning away more refugees who deserve protection. He added that the immigration debate "has become quite irrational in today's world."

He appealed to developed countries to increase their refugee resettlement levels to those of recent years.

Guterres particularly called on the US to accept refugees.

"I strongly encourage the United States government to come back to the levels of resettlement that we witnessed until two or three years ago," he said.

Guterres also described what he sees as fueling the current refugee crisis.

"Political populism, xenophobia, racism, in which refugees become a target, many times being accused of being part of the terror threat when refugees are not terrorists -- they are the first victims of terror, they are fleeing terror; that is why they are refugees," Guterres said.

Trump, who has espoused an "America First" approach to foreign policy, has warned of the threat posed by people trying to get into the US, particularly from mostly Muslim countries. His travel bans are still being debated in US courts.

Guterres said he will travel to Washington next week to discuss issues affecting the UN with members of Congress, as well as Trump's proposed US cuts to UN investment.

When asked about his current relationship with the Trump team, including UN Ambassador Nikki Haley, Guterres said: "I think we always must engage positively and constructively with any administration in the world. But ... the financial question is still to be seen."

Guterres added that he has no plans to meet with Trump personally on this trip.