(CNN) President-elect Donald Trump's transition team has told all ambassadors who are political appointees of President Barack Obama to vacate their overseas posts by Inauguration Day, refusing to provide even brief grace periods for special cases, several senior State Department officials and American diplomats said.

"Political" ambassadors often have close ties to the president or are major donors and almost always leave their post at the end of the term, while career ambassadors frequently remain in their posts.

In keeping with standard practices, the White House requested and received resignations from all politically-appointed chiefs of mission shortly after Election Day, several senior State Department officials told CNN.

But, in a break with precedent to consider grace periods on a case-by-case basis, a subsequent State Department cable sent last month to all non-career ambassadors instructed them to finish their service by January 20 "without exceptions," the sources said.

The move, first reported by The New York Times, leaves the US without top envoys at key posts such as Britain, Germany, Canada, Japan and Saudi Arabia until President-elect Donald Trump nominates successors and they are confirmed, a process that could take months. In the meantime, the posts will be run by the highest-ranking career official until Trump nominates, and the Senate confirms, someone new.

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