An era of relative harmony between the United States and United Nations appears to be coming to an end, as President Trump's envoy takes on the international body's inclination toward inaction -- and U.N. leaders take swipes at the new administration in Washington.

U.S. Ambassador to the United Nations Nikki Haley made clear last week that her tenure at the New York headquarters would mark a change in tone as she defended U.S. missile strikes against the regime of Syrian President Bashar al-Assad following a deadly chemical attack on citizens, including children.

Haley accused the U.N. of "consistently" failing to act on Syria, while also forcing the Security Council into open session when one nation, Bolivia, apparently tried to discuss the strikes in private.

HALEY FORCES BOLIVIA TO DEFEND ASSAD 'ATROCITIES' IN 'PUBLIC VIEW' AT UN SESSION



There are numerous signs that U.N. leaders are bristling at the shift and doing what they can to push back.

Secretary General António Guterres’ decision to retain an economist who frequently lambasts Trump, and a U.N. decision to have Mexico help lead international migration talks, are the latest indications an already frosty relationship with the Trump administration is likely to get colder.

Guterres’ spokesman Stephane Dujarric confirmed to blog Inner City Press this week that Jeffrey Sachs, a world-renowned economist who has served as a senior U.N. adviser since 2002, will continue in that role.

While Sachs has been in the post for many years, he has been particularly vociferous in his criticism of Trump. Last week, he wrote a column called “Donald Trump’s Climate Fantasies” in which he told readers to pity the president.

“In less than 100 days, we have learned that Trump is a man living in a fantasy world. He issues decrees, barks orders, sends out midnight Tweets, but to no avail. The facts – real ones, not his 'alternative' variety – keep intervening,” Sachs wrote.

Also in March, he wrote a column for The Boston Globe accusing Trump of “mining that deep vein of fear and hate” with his travel ban affecting certain Muslim-majority countries. He also has called Trump the “quintessential short-run populist” and a “non-stop font of lies.” Sachs did not respond to a request for comment from Fox News.

Sachs' reappointment is not the only possible anti-Trump move in recent days. According to a letter obtained by Fox News, U.N. General Assembly President Peter Thomson told the Mexican ambassador he has appointed him as a co-facilitator of upcoming intergovernmental negotiations of a “global compact” for safe, orderly and regular migration.

Trump’s relations with the Mexican government have been tense ever since he made his demand that Mexico pay for a wall on the southern border.

Meanwhile, the Trump administration has continued to criticize what it sees as a bloated and biased body. Haley has been particularly sharp in her criticism of the body’s perceived anti-Israel tilt, working to secure the withdrawal of a report last month that called Israel an “apartheid state,” while Trump’s budget blueprint has hinted at deep cuts to U.N. spending.

"We've carried the burden for a really long time,” Haley said recently. “And it doesn't mean we're not going to continue to pay our due. But at some point, other countries have to step in and start funding these missions, too.”

Amid the cost-cutting push, Foreign Policy reported that Haley urged the U.N. to trim the peacekeeping mission to Congo. Haley wanted a cut of 1,500 troops, though only came away with a 370-troop cut. The push, however, signaled the Trump administration sees tightening costs as a real issue – an effort likely to face strong resistance from U.N. officials.

Guterres himself has been taking swipes at Trump on his trips abroad. At an address to the League of Arab States in Jordan last month, Guterres slammed “populist political leaders – [who] for short-sighted and cynical reasons -- distort Islam to spread anti-Muslim hatred, playing into the hands of terrorist and extremist groups.”

“It breaks my heart to see developed countries closing their borders to refugees fleeing this region, and worse, sometimes invoking religion as a reason to keep them out,” he said.

In February, the U.N.’s High Commissioner for Human Rights Prince Zeid Ra'ad Al Hussein took an apparent shot at the Trump administration’s threat to leave the Human Rights Council, and criticized “political leaders who today wage campaigns against universal human rights, or threaten withdrawal from international or regional treaties and the institutions which uphold them.”

After praising the anti-Trump marches that took place the day after Trump’s inauguration, and adding he was “proud members of my staff took part,” the career Jordanian diplomat warned about political actors who would withdraw from parts of the “mulitilateral system.”

“[O]ur rights, the rights of others, the very future of our planet cannot, must not be thrown aside by these reckless political profiteers,” he said.

The warning apparently did not work, as the U.S. boycotted the first session of the council amid concerns about anti-Israel resolutions.