USviewer: "This is a very complicated divorce," Kerry told the Aspen Ideas Festival on Tuesday, one day after he met with British Prime Minister David Cameron.
On Thursday, British citizens voted in a historic referendum to leave the EU, a bloc that the UK joined more than 40 years ago.
The decision sent shockwaves around the world and prompted Cameron to announce his resignation.
Kerry said that Cameron "feels powerless -- and I think this is a fair conclusion -- to go out and start negotiating a thing that he doesn't believe in and he has no idea how he would do it.”
"And by the way, nor do most of the people who voted to do it," he said, in an apparent reference to "Leave" campaigners such as former London mayor Boris Johnson, who is now the frontrunner to succeed Cameron as premier.
Kerry also said that London does not want to end up feeling boxed in after two years without a new association agreement and having to leave the EU without one.
Asked whether the Brexit decision could be "walked back," Kerry said, "I don't as secretary of state want to throw them out today. I think that would be a mistake. But there are a number of ways."
In the June 23 referendum, about 52 percent of British voters opted to leave the EU, while roughly 48 percent of the people voted to stay in the union. More than 17.4 million Britons said the country should leave the bloc, as just over 16.14 million others favored remaining in the EU.
According to presstv, Those in favor of a British withdrawal from the EU argued that outside the bloc, London would be better positioned to conduct its own trade negotiations, better able to control immigration and free from what they believe to be excessive EU regulations and bureaucracy.
Those in favor of remaining in the bloc argued that leaving it would risk the UK's prosperity, diminish its influence over world affairs, and result in trade barriers between the UK and the EU.