Athens, Greece (CNN) President Barack Obama issued a warning Tuesday against "crude nationalism" in the wake of Donald Trump's victory, saying during a news conference on his final overseas trip that the President-elect had tapped into a "troubling" strain of rhetoric playing on Americans' fears of globalization to win the presidency.

Speaking in Athens, Obama said he recognized an "anger and fear in the American population" over threats of mechanization and globalization, but that Republican officials didn't use facts when making their case about the US economy.

"You've seen some of the rhetoric among Republican elected officials and activists and media. Some of it pretty troubling and not necessarily connected to facts, but being used effectively to mobilize people," Obama said at a news conference alongside Greek Prime Minister Alexis Tsipras. "And obviously, President-elect Trump tapped into that particular strain within the Republican Party and then was able to broaden that enough and get enough votes to win the election."

Obama said countries across Europe, as well as the United States, were confronting populist movements based on a fear of intruding global forces, arguing people "are less certain of their national identities or their place in the world." He said leaders should heed lessons from results in the US and in Britain, which in June voted to exit the European Union.

"It starts looking different and disorienting. And there is no doubt that has produced populist movements, both from the left and the right," he said. "That sometimes gets wrapped up in issues of ethnic identity or religious identity or cultural identity. And that can be a volatile mix."

He said Americans must guard against those trends during Trump's presidency, and insisted he, too, would speak out against divisive language even after he leaves office.

"We are going to have to guard against a rise in a crude sort of nationalism or ethnic identity or tribalism that is built around an us and a them," he said. "And I will never apologize for saying that the future of humanity and the future of the world is going to be defined by what we have in common as opposed to those things that separate us and ultimately lead us into conflict."

Explaining Trump's win to Europeans, who themselves face rising populist movements in countries across the continent, Obama theorized Americans were simply ready for a change in Washington and were not rejecting his policies outright.

"People seem to think I did a pretty good job," he said, alluding to surveys showing his job approval rating near its highest point. "And so there is this mismatch, I think, between frustration and anger. Perhaps the view of the American people was just that need to shake things up."

Obama is on a week-long, three-stop foreign swing that has taken new urgency following Trump's election. He's expecting questions from leaders in Greece, Germany and Peru about Trump's motives and his plans going forward.

Global leaders have expressed concerns over Trump's suggestion on the campaign trail that the US might not keep its current level of international commitments, including to NATO. Obama Tuesday also called Greece a "reliable ally" in its commitment to the transatlantic alliance, even under tremendous strain from the country's debt crisis.

"We are proud to count Greece as one of our closest allies and one of our greatest friends," Obama said at the news conference, commending the country's ability to meet its financial obligations to NATO despite austerity measures.

Greek debt crisis

Air Force One touched down at the Eleftherios Venizelos International Airport just past 3:30 a.m. ET (10:30 a.m. local time) on Tuesday, where Obama was greeted with a red carpet and an ornate display of military pageantry.

In the morning preceding his news conference, Obama paid a courtesy call to Greek President Prokopis Pavlopoulos.

Obama also sat for formal talks with Tsipras at the neo-classical presidential mansion in central Athens.

Ahead of the meeting, Obama had said he would stress that debt reduction strategies beyond austerity must be utilized in Europe going forward.

"I will continue to emphasize our view that austerity alone cannot deliver prosperity," Obama said, traveling with Treasury Secretary Jack Lew, who was expected to discuss Greece'spersistent debt crisis with officials here.

Obama is the first US president to visit Greece in 17 years, and he focused intently on the debt situation and the influx of refugees fleeing the Syrian civil war.