Britain’s crisis deepened Monday as British and EU officials scrambled to deal with the shock of British voters’ decision to leave the European Union, and ease both anger and the uncertainty that has plunged the country into what some describe as its worst disarray since World War II.

U.S. Secretary of State John Kerry stopped in London Monday and reaffirmed what he said is an unbreakable bond between the United States and Britain.

Kerry assured Britain the special relationship between the two countries will not change because of the Brexit decision. Speaking after a meeting Monday with British Foreign Secretary Philip Hammond, the top U.S. diplomat said that relationship is “perhaps even more important in these days of questioning on behalf of many people.” Ties, he said, remain “strong and crucial.”

British officials spent Monday working to contain the aftermath of the vote that sent markets and the British pound tumbling for another day. In his first remarks to parliament since the vote, Prime Minister David Cameron said the British economy is strong enough to deal with the turbulence. “It is clear that markets are volatile. There are some companies considering their investments and we know this is going to be far from plain sailing,” he said.

Earlier, in Brussels, Kerry appealed for calm amid the anxiety that British voters’ decision has caused in Europe, and in global markets around the world.

He also cautioned EU members against punishing Britain for its decision.

“It’s absolutely essential that we stay focused on how, in this transitional period, nobody loses their head, nobody goes off half-cocked, people don’t start ginning up scatterbrained or revengeful premises,” Kerry said.