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President Trump will host members of the United Nations Security Council at the White House Monday, a highly unusual meeting made even more startling because of his harsh criticism of the international institution during the campaign and since taking office.

U.S. Ambassador to the UN Nikki Haley is serving this month as the President of the Security Council, a role that rotates each month among the 15 members of the group — including the five permanent members: the U.S., Great Britain, France, China and Russia. Other members of the group right now include Egypt, Japan, Senegal, Bolivia, Ethiopia, Italy, Kazakhstan, Sweden, Ukraine and Uruguay — although non permanent members do not have veto power over Security Council resolutions.

Haley will be attending before the group returns to New York for scheduled Security Council meetings on Tuesday.

The president's budget outline proposed deep cuts in the U.S. contribution to the UN, which could dramatically impair its peacekeeping functions around the world.

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Other high profile UN functions include refugee relief and vetting of refugee visa applicants to the U.S, the World Health Organization, UNICEF and the International Atomic Energy Agency in Vienna — the weapons inspectors who monitor Iran's compliance with the nuclear deal.

Diplomatic sources told NBC News the ambassadors are expecting to have coffee at Blair House — also known as the The President's Guest House — with members of Congress Monday morning and then go to the White House to meet with the President and have lunch.

North Korea will inevitably be a major point of discussion.

China abstained on a UN resolution last week condemning the latest missile test — instead of vetoing it — a symbolic gesture. But Beijing has so far resisted tougher action.

The Trump administration could unilaterally impose much tougher banking sanctions against North Korea if it wanted to — similar to the Obama administration's past sanctions on Iran — for instance blocking all foreign banks who deal with North Korea from trading in dollars or banking in the U.S. That would be a direct hit on China's financing of the regime in Pyongyang.

So far, however, the Trump White House has not chosen that route but has repeatedly said "all options are on the table," implying military action was possible.

Many experts, including former Defense Secretary Leon Panetta, have discounted the viability of preemptive military strikes given the proximity of millions of people in Seoul and 28,500 U.S. troops in South Korea, all within artillery range of North Korea if it were to retaliate.

This all comes as an American citizen, a Korean-American accounting instructor, was detained Sunday at the airport in Pyongyang while trying to leave the country after having been there for a month.

The State Department has reached out to Sweden's embassy, the protectorate for the U.S. in North Korea, to try to obtain his release.

Editor's Note: A story on April 23 incorrectly listed the number of members of the United Nations Security Council.