European Council president Donald Tusk has urged Theresa May to start the Brexit process "as soon as possible", saying: "the ball is now in your court".
The leaders of the other 27 EU nations will hold talks in Bratislava next week and Mr Tusk said they would "discuss the political consequences of Brexit" for Europe.
But as they met in Downing Street, he told Mrs May: "It doesn't mean that we are going to discuss our future relations with the UK in Bratislava, because for this - and especially for the start of the negotiations - we need the formal notification, I mean triggering Article 50.
"I'm aware that it is not easy but I still hope you will be ready to start the process as soon as possible."
The prime minister said she wanted a "smooth" Brexit process and told Mr Tusk they had "serious issues" to discuss.
More top news
The outage struck Hartfield-Jackson Atlanta International Airport at around 1pm local time, stopping all outgoing flights.
It will be a chilly day for many parts with the highest temperatures, about 10 Celsius (50F) in the far northwest.
The social site has been overhauling many of its policies in recent weeks following repeated criticism over how it handles abusive content.