Top US diplomat John Kerry will fly to Brussels and London on Monday for talks with Washington's key allies in the aftermath of Britain's vote to leave the European Union.
US officials travelling to Rome with the secretary of state told reporters on the flight that two stops had been added to his European itinerary at the last moment.
"Tomorrow we'll go to Brussels and then on to London," one official said.
Washington was dismayed last week when British voters chose to leave the European Union, a decision that triggered global economic uncertainty and fears other EU members will follow suit.
President Barack Obama had made clear his concern about the Brexit referendum, and now US officials are scrambling to try to stop the political crisis harming Western unity.
Kerry arrived in Rome on Sunday on a planned visit to have lunch with Foreign Minister Paolo Gentiloni and a working dinner with Israeli Prime Minister Benjamin Netanyahu.
But officials on his flight said that on Monday he would fly on to the EU headquarters in Brussels for crisis talks with EU foreign policy chief Federica Mogherini.
Mogherini had been expected to meet Kerry in Rome on Sunday, but she was busy dealing with the fallout of the dramatic vote, which stunned European and world leaders.
From Brussels Kerry will continue to London to see Foreign Secretary Philip Hammond and other officials from outgoing British Prime Minister David Cameron's government.
The two senior diplomats are expected to hold a joint news conference before Kerry leaves to return to Washington.
Obama and Kerry have been at pains to insist the vaunted "special relationship" between Washington and London will survive what US officials view as the Brexit debacle.
But Washington foreign policy experts are all but unanimous in assessing that the White House will increasingly turn to core EU allies to defend its interests on the continent.
Obama himself, on a visit to London last month, warned British voters that their nation would go "to the back of the queue" for a US trade deal if they voted "out."
US officials are also keen to help London's divorce from Brussels go through smoothly without further inflicting further damage on skittish world financial markets.
But they, like many EU capitals, are also concerned not to allow Brexit to serve as an inspiration for eurosceptic forces in other members such as Italy or The Netherlands.
The London visit will be the first by a senior US official since Thursday's dramatic referendum, when voters demanded Britain leave the world's richest trading bloc.
Kerry's trip had originally been planned as an opportunity to meet Netanyahu and discuss regional security and the stalled Israel-Palestinian peace process.
But US officials played down the chance of any concrete progress, insisting that the pair meet regularly and that no new initiative would be on the table.