Athens - The NATO alliance is "absolutely vital" to US interests, President Barack Obama said Tuesday, as he flew in to Europe to reassure jittery allies concerned by Donald Trump's shock presidential election win.
Speaking in Athens on the first leg of his last foreign trip as leader, Obama stressed that a strong Europe was "good for the world and the US", after Trump appeared to downplay the importance of historic transatlantic ties.
Europeans, especially in eastern countries closest to Russia's orbit, have been shaken after Trump appeared to call into question Washington's near 70-year security guarantee by saying he would only help NATO allies if they paid their way.
But Obama stressed that the transatlantic relationship was the "cornerstone of our mutual security as well as prosperity" and that was the case regardless of who was sitting in the Oval Office.
"Across Democratic and Republican administrations there is a recognition that the NATO alliance is absolutely vital," he said.
As Obama touched down in Athens, NATO head Jens Stoltenberg said he was confident Trump would live up to US commitments to the alliance.
"I am certain that he will be a president... who will live up to all the commitments of the United States in the alliance, because a strong NATO is important for Europe but it's also important for the United States," Stoltenberg said.
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- Challenge of globalisation -
The fourth president to travel to Greece, Obama will visit the Acropolis before delivering a speech on the challenges of globalisation.
He is also holding talks with Greek President Prokopis Pavlopoulos and Prime Minister Alexis Tsipras.
Security was tight in central Athens, with thousands of police officers on the streets and traffic set to be disrupted for several hours.
On Wednesday, Obama travels to Germany, where he will speak with close ally Chancellor Angela Merkel as well as the French and British leaders.
He concludes his trip with a stop in Peru for a summit of the Asia-Pacific Economic Cooperation forum (APEC) where he is expected to meet Chinese President Xi Jinping.
Meanwhile, Trump was set for another meeting with running mate Mike Pence to discuss cabinet appointments amid reports of intense infighting over choice posts.
Trump appears torn between a campaign promise to shake up Washington and the need to build a cabinet with political experience and connections with Congress.
Possible candidates for secretary of state include hawkish former diplomat John Bolton and former New York mayor Rudy Giuliani, US media reported.
Retired General Michael Flynn is a possible national security advisor pick, while Republican Senator Jeff Sessions could be defence secretary or attorney general, according to CNN.
Speaking to reporters on the eve of the trip, Obama cautioned his brash successor that he faces a reality check if he tries to enact some of his more controversial campaign promises.
"This office has a way of waking you up," Obama said.
- Suspicious of US -
Greek leaders are anxious for a new US pledge to help alleviate the country's enormous public debt, a measure actively sought by the International Monetary Fund but opposed by leading European lender Germany.
Greek government spokesman Dimitris Tzanakopoulos said Obama's visit would "contribute to promoting a fair and viable solution on the Greek debt".
Obama is also expected to address Europe's challenge in accommodating hundreds of thousands of refugees who have fled conflict in the Middle East and elsewhere.
Athenians voiced mixed feelings about his visit.
"We live in the hope that it will bring something positive for Greece and the region," said Spyros, a 30-year-old civil servant.
Ekaterini Tsarmoutzi, a private employee, said Obama "was nice as a person, but above all he is an American".
Many Greeks are suspicious of the United States after it helped install a repressive dictatorship in the country in the 1960s.
Greek trade unions, leftist and anarchist parties have called for protests to denounce "imperialist" US involvement in wars in the Middle East.
"Butcher Obama is not wanted," the Communist-affiliated PAME union said, while the left-leaning Efimerida ton Syntakton charged that Obama's drone attacks have killed 4,700 people.
Police have banned demonstrations in areas of the capital where Obama will hold his meetings.
The increased security also comes after assailants threw a grenade at the French embassy in Athens last week, slightly wounding an officer.
Several Athenians questioned the possible benefits of a visit by an outgoing president.
"He's just here for a stroll. Somebody else is taking over, and from a different political party at that, so there are no commitments binding (on his successor)," said pensioner Costas Bousgos.