U.S. President Donald Trump has picked Kelly Craft, the U.S. ambassador to Canada, as his nominee to serve as the next U.S. ambassador to the United Nations.

Trump said in a pair of tweets that Craft "has done an outstanding job representing our Nation" and he has "no doubt that, under her leadership, our Country will be represented at the highest level."

A source with direct knowledge of the situation told CBC News that officials in Canada will be sad to see Craft leave because she had Trump's ear and could get almost anyone on the phone or in the room if she wanted to.

The source said Craft has been a "tremendous" ambassador and proved herself "a great friend to Canada."

Two people familiar with the matter who spoke on condition of anonymity told The Associated Press that Trump had been advised that Craft's confirmation would be the smoothest of the three candidates he had been considering to fill the job last held by Nikki Haley.

Senate majority leader Mitch McConnell, a Republican from Kentucky, had backed Craft for the post, and she also has the support of Secretary of State Mike Pompeo and national security adviser John Bolton, the people said. Trump's first pick to replace Haley, State Department spokesperson Heather Nauert, withdrew from consideration last weekend.

....Kelly has done an outstanding job representing our Nation and I have no doubt that, under her leadership, our Country will be represented at the highest level. Congratulations to Kelly and her entire family! —@realDonaldTrump

Craft, a Kentucky native, was a member of the U.S. delegation to the UN General Assembly under former President George W. Bush's administration. She is also friends with McConnell's wife, Transportation Secretary Elaine Chao, and thanked Chao for her "longtime friendship and support" at her swearing-in as ambassador.

As U.S. ambassador to Canada, she played a role in facilitating the U.S.-Mexico-Canada trade agreement, a revamp of the North American Free Trade Agreement.

Not the job she wanted: source

The source with direct knowledge of the move told CBC News that the UN posting is not a gig that Craft had asked for or really even wanted.

In Canada, she can operate at arms length from the U.S. administration and she had concerns about the prospect of daily interactions with the White House and taking orders from Jared Kushner or Bolton.

While she may not have wanted the position, she was advised by those close to her that if Trump asks her to take the job she should do so, the source told CBC News.

Canadian Prime Minister Justin Trudeau meets with Craft on Parliament Hill in Ottawa in November 2017. (Sean Kilpatrick/Canadian Press)

Trump had also considered U.S. Ambassador to Germany Richard Grenell and former U.S. Senate candidate John James of Michigan for the post.

Nauert's withdrawal from consideration came amid a push within the administration to fill the position, given a pressing array of foreign policy concerns in which the United Nations, particularly the UN Security Council, is likely to play a significant role. From Afghanistan to Venezuela, the administration has pressing concerns that involve the world body, and officials said there had been impatience with the delays on Nauert's formal nomination.

Trump said Dec. 7 that he would pick the former Fox News anchor and State Department spokesperson for the UN job, but her nomination was never formalized. Notwithstanding other concerns that may have arisen during her confirmation, Nauert's nomination had languished in part due to the 35-day government shutdown that began Dec. 22 and interrupted key parts of the vetting process. Nauert cited family considerations in withdrawing from the post.

Watch: Craft calls on China to released detained Canadians:

Ambassador Kelly Craft called China's detention of two Canadians 'unacceptable' in her first public comments about the cases this week. 7:57

With Nauert out of the running, officials said Pompeo had been keen on Craft to fill the position. Although Pompeo would like to see the job filled, the vacancy has created an opportunity for him and others to take on a more active role in UN diplomacy. On Thursday, for example, Pompeo was in New York to meet with UN chief Antonio Guterres.

Trump has demoted the UN position to sub-Cabinet rank, in a move backed by both Bolton and Pompeo, according to three other officials. Grenell had suggested he wasn't interested in a non-Cabinet role. The officials were not authorized to discuss internal personnel deliberations and spoke on condition of anonymity.

Haley had been a member of the Cabinet and had clashed repeatedly with former Secretary of State Rex Tillerson and others during the administration's first 14 months. Bolton was not a Cabinet member when he served as UN ambassador in George W. Bush's administration, and neither he nor Pompeo is eager to see a potential challenge to their foreign policy leadership in White House situation room meetings, according to the officials.

Craft is married to billionaire coal-mining executive Joe Craft, and they are major Republican donors.