U.S. Ambassador to Canada Bruce Heyman has resigned amid news that president-elect Donald Trump's transition team has issued a blanket mandate requiring politically appointed ambassadors installed by Barack Obama to leave their posts by Inauguration Day.

"As requested I have resigned as US Ambassador to Canada effective 1/20," Heyman announced on Twitter.

Heyman's tweet followed a similar announcement made earlier by U.S. ambassador to New Zealand Mark Gilbert. "I will be departing on January 20th," Ambassador Mark Gilbert said in a Twitter message to Reuters.

The mandate was issued "without exceptions" through an order sent in a State Department cable on Dec. 23, Gilbert said.

He was confirming a report in the New York Times, which quoted diplomatic sources as saying previous U.S. administrations, from both major political parties, have traditionally granted extensions to allow a few ambassadors, particularly those with school-age children, to remain in place for weeks or months.

Officials from the State Department and Trump's transition team were not immediately available for comment. The order threatens to leave the United States without Senate-confirmed envoys for months in critical nations like Canada, Germany and Britain, the New York Times reported.

A diplomatic gap wouldn't be without precedent for Canada in modern times; in fact, it last happened after the previous change in administration between Republicans and Democrats.

David Wilkins, appointed ambassador to Canada by George W. Bush, served his tenure right up until the inauguration of Barack Obama in January 2009. Confirmation hearings for David Jacobson then took place in August of that year and he officially began as ambassador of Canada in October.

There was also a gap of six months before Bill Clinton appointee James Blanchard took his post in 1993 after Peter Teeley, tapped by George H.W. Bush, left the job.

In contrast, the posting to Ottawa was only officially vacant for a week before Paul Cellucci began serving in 2001, after George W. Bush and the Republicans assumed control of the White House from the Democrats.

Heyman has served as ambassador to Canada since April 2014.​

A senior Trump transition official told the newspaper there was no ill will in the move, describing it as a simple matter of ensuring Obama's overseas envoys leave the government on schedule, just as thousands of political aides at the White House and in federal agencies must do.

Trump has taken a strict stance against leaving any of Obama's political appointees in place as he prepares to take office on Jan. 20, aiming to break up many of his predecessor's signature foreign and domestic policy achievements, the newspaper said.

Diplomats told New York Times the order has thrown their personal lives into a tailspin, leaving them scrambling to secure living arrangements and acquire visas allowing them to stay in their countries so their children can remain in school.