The United States is reacting cautiously to the decision by Britain's voters to bolt the 28-nation European Union, with a White House official saying only that President Barack Obama is being kept up to date on developments.

The official, who insisted on speaking anonymously because of the ongoing events overseas, said Obama was expected to speak with Prime Minister David Cameron "over the course of the next day."

"We will release further comment as soon as possible," the official's statement said.

Obama has encouraged Britain to remain in the EU but has said the decision ultimately was up to the voters.

Before Obama visited the United Kingdom in April, he wrote an op-ed for the Telegraph newspaper encouraging it to remain in the EU. He said Great Britain's presence "magnifies" its influence and helps spread "British values." The president said in the op-ed that the close relationship between the two countries would endure, no matter how the vote went.

The White House statement came while Obama was in San Francisco the evening before he was to address aspiring entrepreneurs at Stanford University. The statement came with global markets diving as investors reacted with alarm to the historic vote.