US President Barack Obama said on Saturday he expected Britain to go through with leaving the European Union after last month's referendum and was concerned to limit the damage to the British, European and global economies from the move.

As a friend, ally and trading partner of Britain and of the EU, he said Washington wanted to see an orderly negotiating process and as close a relationship as possible in future.

"It's important that neither side harden positions in ways that ultimately do damage to their respective economies and ultimately to the world economy at a time when our world economy is still pretty wobbly in places," Obama told a news conference at the end of a NATO summit in Warsaw.