US Secretary of State John Kerry steps off his plane after arriving at Kastrup International Airport in Copenhagen Thomson Reuters Secretary of State John Kerry on Sunday urged Britain and the European Union to manage their divorce responsibly for the sake of global markets and citizens, a day before he was to become the first senior American official to visit London and Brussels since the United Kingdom's historic referendum.

Kerry emphasized the importance of thoughtful cooperation at a time of economic uncertainty and fears about crumbling European unity. He said he would bring a message of support to both capitals. But he offered no concrete suggestions for how the two sides should make good on the decision by British voters to leave the 28-nation bloc.

"The most important thing is that all of us, as leaders, work together to provide as much continuity, as much stability, as much certainty as possible," Kerry said as he met in Rome with Italian Foreign Minister Paolo Gentiloni.

Responsible handling of the situation, he said, will help "the marketplace understand there are ways to minimize disruption, there are ways to smartly move ahead in order to protect the values and interests that we share."

Kerry had scheduled talks in Rome with Israeli Prime Minister Benjamin Netanyahu later Sunday. But confronted with the gravity of Britain's vote on Thursday, which crushed markets from the to Japan, Kerry set up a frantic, four-nation schedule for Monday.

After gathering again with Netanyahu in the morning, he planned to fly to Brussels to discuss Europe's situation with EU foreign policy chief Federica Mogherini. Later, he was to meet British Secretary Philip Hammond in London before returning to Washington by day's end.

With the British, Kerry intended to echo last week's immediate response, which focused on the unchanged nature of the allies' "special relationship."

German Chancellor Merkel gives a statement in Berlin Thomson Reuters

Even the gloomiest of predictions about the British exit from the EU do not foresee the collapse of the close cultural ties or military alliance between Washington and London. But how relations evolve is an open question, especially if Britain's separation from the EU causes significant economic pain in the United States.

Visiting Britain in April, President Barack Obama noted ongoing -EU trade negotiations and warned Britons that a vote to "leave" could put them at the back of the line for similar deals. Since the result of the referendum, however, Obama and other American officials have gone out of their way to emphasize the durability of the relationship, playing down the idea of any repercussions from Washington.

"We will continue, the United States, to have a very close and special relationship with Great Britain," Kerry said on the rooftop of a hotel overlooking the Pantheon. "We value that relationship. That does not change because of this vote."

In Brussels, Kerry planned to emphasize backing for the EU amid speculation that other member countries could follow Britain's lead.

Alongside Gentiloni, Kerry stressed the importance of political unity among 27 remaining countries that still represent a market of 450 million people, and help the provide security to unstable places in North Africa and the Middle East, and far-flung areas of conflict such as Afghanistan.

EU flags fly outside the European Commission headquarters in Brussels, during gas talks between the EU, Russia and Ukraine on Oct. 30, 2014. Francois Lenoir/Reuters

It's unclear what more Kerry, or the , can say or do right now to help Britain or the EU.

Britain's exit negotiations could be a complicated, protracted affair, and the Americans are likely to have little say in the matter. The also has no answer for the EU's dilemma about how to respond to the first loss of a member in its history.

"There are steps Europe needs to take to respond to the expression of voters and the concerns of people in other countries," Kerry said, without entering the European debate over a quick or a slow breakup with Britain. He described the -EU partnership as critical for Europe, America and the world.