In return, Mr Fallon told Ash Carter that the UK would remain committed to tackling global security issues and continue in the fight against Islamic State (IS) as a member of Nato.
The pair spoke by phone on Friday after it was announced Britain had voted to sever ties with Brussels.
“The secretary reaffirmed that those bonds endure after yesterday’s vote by the United Kingdom to exit the European Union,” Pentagon spokesman Peter Cook said.
Mr Carter emphasised the UK’s role as a “vital” member of Nato and welcomed Mr Fallon’s assurances that Britain will continue to play an active role in global security issues.
Mr Cook said: “The two leaders committed to work together to strengthen international partnerships that have helped ensure security for the United States, the United Kingdom, Europe and the world.
“We will see that close work together continues in next month’s Nato summit in Warsaw, and it’s on display every day in the fight against Isil (IS).”