UNITED NATIONS (Reuters) - United States interests, including national security, would be harmed if the country retreats from a leading role at the United Nations, U.S. Ambassador Samantha Power warned on Thursday amid a backlash among Washington lawmakers against the world body.

United States Ambassador to the United Nations Samantha Power addresses media following a United Nations Security Council vote, aimed at ensuring that U.N. officials can monitor evacuations from besieged parts of the Syrian city of Aleppo, at the United Nations in Manhattan, New York City, U.S., December 19, 2016. REUTERS/Andrew Kelly

President-elect Donald Trump also disparaged the United Nations after the Security Council adopted a Dec. 23 resolution demanding an end to settlement building by U.S. ally Israel. Trump questioned the value of the organization, while some Republican lawmakers threatened to cut crucial U.S. funding.

In an exit memo on Thursday, Power - a member of President Barack Obama’s cabinet - wrote: “Other nations will follow us if we continue to lead; without our leadership the vacuum on the global stage will prove very harmful to U.S. interests.”

Among the reasons for continued strong U.S. engagement at the 193-member United Nations, Power listed North Korea’s nuclear ambitions, conflicts in Syria, Libya and South Sudan, climate change, the global refugee crisis and Russia.

“As Russia continues to menace our allies and attempt to interfere in political systems in Europe and beyond, we will need to show broad condemnation of these actions in U.N. fora,” Power wrote in the 13-page memo.

U.S. intelligence agencies say Russia was behind cyber attacks before the U.S. election in November that aimed to help Trump defeat Democrat Hillary Clinton. Moscow denies this.

Power also promoted the importance of the United Nations in ensuring the success of the a deal between Iran and key world powers to curb Tehran’s nuclear ambitions in exchange for relief from economic sanctions. Trump has vowed to scrap the deal.

“We must continue to fulfill our own commitments and use U.N. Security Council Resolution 2231 ... to ensure that Iran’s nuclear program will remain exclusively peaceful,” Power said.

Trump plans to replace Power with South Carolina Governor Nikki Haley once she is confirmed by the U.S. Senate.

“Working with the U.N. to address these challenges will not be a litmus test of whether one is committed to international norms and institutions or not – it will simply be a strategic necessity,” said Power, who has been ambassador since 2013.

Trump last week described the world body as “just a club for people to get together, talk and have a good time.”

New U.N. Secretary-General Antonio Guterres spoke with Trump on Wednesday and the two had “a very positive discussion on U.S./U.N. relations,” a U.N. spokesman said.