Joe Biden used his first phone call with Boris Johnson as U.S. president-elect to warn the British leader not to compromise peace in Northern Ireland in his pursuit of Brexit.

During the course of a 20- to 25-minute conversation on Tuesday, Biden “reaffirmed his support” for the 1998 deal that put an end to the violence in Northern Ireland, according to a statement from the president-elect’s team.

A British official confirmed that Biden raised the Good Friday Agreement in the context of Brexit negotiations, and that Johnson responded by promising the president-elect that Britain would uphold the peace accord.

Biden spoke later to Irish Prime Minister Micheal Martin, and again made a point of emphasizing his backing for peace in the region.

The exchange with Johnson marks an uncomfortable start to the newest incarnation of the so-called special relationship between the U.K. and the U.S. and suggests Biden will not be an unequivocal backer of Johnson’s Brexit project in the way President Donald Trump has been.

The two men have never met in person, and Biden has shown himself to be fiercely proud of his Irish roots. His transition to power comes at a delicate phase in the U.K.’s divorce from the European Union. Unlike Trump, Biden was opposed to Brexit and has aired his views on it.

Known History

Back in September, Biden raised his concerns over Johnson’s plan to break international law by reneging on parts of the Brexit divorce agreement he struck with the EU relating to trade with Northern Ireland.

President Barack Obama, who Biden served under, famously weighed in on the 2016 referendum to say that if the U.K. opted out of the EU the U.S. would be put at the “back of the queue” when it came to trade talks.

Those talks, begun under Trump, will now likely be followed up by a Biden administration in a process that could take years, judging by the length of previous trade negotiations.

Biden had said in September that any U.S. trade deal with the U.K. must be “contingent” on respecting the Northern Ireland peace agreement and avoiding a return to a hard border with the Republic of Ireland.

A British official said the call was friendly and good. The U.K. has always insisted its plans do not compromise peace in the region and are intended to safeguard stability.