Barack Obama is saying a final official goodbye to Europe with a trip to Germany.
The outgoing US president is in Berlin to discuss extending sanctions on Russia for its intervention in Ukraine.
He and German Chancellor Angela Merkel are also expected to consider imposing new sanctions for Russian bombing over Syria.
The discussions come amid doubts about the future of Washington-enforced penalties after US President-Elect Donald Trump said he would seek closer ties with Moscow.
Speaking in Greece ahead of the Berlin visit, Obama sought to reassure allies of America’s commitment to core strategic interests worldwide.
“In recent years we made historic investments in NATO, increased America’s presence in Europe and today’s NATO, the world’s greatest alliance, is as strong and as ready as it has ever been,” he said in Athens.
“And I am confident that just as America’s commitment to the transatlantic alliance has endured for seven decades, whether it’s been under a Democratic or a Republican administration, that commitment will continue, including our pledge and our treaty obligation to defend every ally.”
One German official told Reuters the situation was critical. He said the US and Europe had to “prevent a situation where the EU rolls over the sanctions and then the new US president comes in and lifts them.”