He was speaking at the G20 summit
Barack Obama has backed up his pre-Brexit referendum claim that there will be no special favours for the UK in relation to a trade deal if they vote to leave the EU.
Theresa May has welcomed the economic reaction to Brexit - as Barack Obama warned there would be no favours for the UK over a trade deal.
Speaking to reporters en route to the G20 summit in China, the British PM said the economic reaction to the EU vote was better than expected.
"We've seen figures giving some different messages in relation to the economy and I think the reaction of the economy has been better than some had predicted post the referendum," she said.
"Now I'm not going to pretend it will be plain sailing. There'll be some difficult times ahead," she added.
The visit is May's first to China, and one of her first tasks was to hold a joint news conference with President Obama.
In it, Mr Obama spoke ominously about the potential for the "special relationship" between the UK and the US to "unravel".
"We're going to do everything we can to make sure that the consequences of the decision don't end up unravelling what is already a very strong and robust economic relationship that can become even stronger in the future," he said.
The US has been discussing a broad EU trade deal and said ahead of the Brexit vote that Britain would be at the "back of the queue" for a trade deal.
However, Mr Obama told the news conference he never said Britain would be punished, but made it clear current negotiations would not be shelved in favour of talks with the UK.
G20 Summit: Obama says no quick post-Brexit trade deal with UK - International Business Times UK https://t.co/UaYCN9qfo1— Marijhaa (@Marijhaa) September 4, 2016