The U.S. has criticized a formal extradition request from the UK for the wife of an American diplomat, who left the country after being involved in a car accident that killed a teenager, as "highly inappropriate" in the latest stage of a brewing row between London and Washington.

The case has been a political headache for UK Prime Minister Boris Johnson, who is cultivating trade relations with Washington in a bid to offset potential economic damage from Brexit.

What happened?

Briton Harry Dunn, 19, died on August 27 when his motorbike collided with a car driving on the wrong side of the road near an air base in Northamptonshire, central England, which is used by the U.S. military as a communications hub.

Sacoolas, believed to be the wife of a U.S. intelligence officer, admitted in October to being the driver, but has cited immunity while refusing to return to Britain to face justice.

She left the UK shortly after the crash and returned to the United States.

The U.S. State Department argued her extradition would be "an abuse" because she had diplomatic immunity, but the case has stirred up debates over the limits of such immunity in cases unrelated to national security.

What does the UK want?

The British Home Office on Friday submitted a formal extradition request for Sacoolas.

"Following the Crown Prosecution Service's charging decision, the Home Office has sent an extradition request to the United States for Anne Sacoolas on charges of causing death by dangerous driving," a Home Office spokesman said. "This is now a decision for the U.S. authorities."

Last month the State Department said it was "disappointed" Britain had charged Sacoolas, asserting that in Washington's view she does enjoy diplomatic immunity in the case. 

In a statement on Friday it again claimed that at the time of the accident and for all of her stay in Britain, Sacoolas had diplomatic immunity. "It is the position of the United States government that a request to extradite an individual under these circumstances would be an abuse," the department said.

"The use of an extradition treaty to attempt to return the spouse of a former diplomat by force would establish an extraordinarily troubling precedent," it said, adding that U.S. remains "engaged with the UK government to look at options for moving forward."

Dunn family spokesman Radd Seiger said the extradition request was a "monumentally large step in the right direction" and that the grieving parents were "happy" and "relieved" at the development.

He added it was "100 percent" sure that Sacoolas would return to Britain.

U.S. President Donald Trump has called the crash a "terrible accident," saying it was common for Americans in Britain to have a hard time driving on the left side of the road.

"When you get used to driving on our system and then you're all of a sudden on the other system where you're driving - it happens," he told reporters in October. "You have to be careful, very careful."

(With input from agencies)