Barack Obama will Wednesday invoke the Cold War history of German-US solidarity, on a long-awaited first visit to Berlin as president, but will face sharp questions on US spy snooping programmes.

Almost 50 years to the day since John Kennedy declared "Ich bin ein Berliner" and 26 years since Ronald Reagan exhorted "Tear down this wall!" Obama will argue that a new generation must muster for history's fresh challenges.

He will also hold talks with German Chancellor Angela Merkel, with whom he usually has respectful relations, but who is pointedly demanding details on the exact extent of US spy agency surveillance programmes.

Though he remains popular in Germany, Obama will struggle to meet the expectations he spun for himself as a presidential candidate, in a speech to 200,000 Berliners in 2008 that made him a political star in Europe.

Since that call for a joint US-European bid to "remake the world" by battling terrorism, global warming, Middle East violence and poverty, Obama has learned the power of the status quo at home and abroad to thwart change.

But frustration will not temper his rhetoric, according to US deputy national security advisor Ben Rhodes.

"Any time a US president speaks in Berlin, it's a powerful backdrop to our post-war history," said Rhodes.

"This is a place where US presidents have gone to talk about the role of the free world.

"With that historical backdrop ... sometimes it's easy to think that history is behind us, essentially. The Wall is down. There's not a threat of global nuclear war. The threats that we do face are far more distant.

"The overarching point that he's going to make is the exact same level of citizen and national activism that was characterized in the Kennedy speech and in the Cold War needs to be applied to the challenges we face now."

In his meeting with Merkel, Obama is under intense pressure to explain the reach and scope of US National Security Agency spying programmes which hoover up data from phone records and the Internet in the United States and abroad.

The programmes, which have special resonance in a nation where snooping operations by the communist Stasi secret police are a painful memory, have triggered alarm across the political spectrum in Berlin.

"I will call for more transparency," said Merkel, who grew up in the communist East in an interview on Monday, adding that Germans wanted to know if their online habits were being spied on by the NSA.

"We have to be clear -- what is being used, what is not being used," she said.

Obama, who arrived in Berlin on Tuesday from the G8 summit in Northern Ireland has said he welcomes public debate on the trade-offs inherent between protecting privacy and citizens from the threat of terrorism.

He will come prepared to discuss the US programmes, but it is unclear whether he will be able to publicly satisfy German curiosity, given that his explanations to Americans on the programs lack specificity.

In his most expansive comments on the NSA's conduct, revealed in leaks to the Guardian and Washington Post, Obama said that no US spies were listening to phone calls or copying down web surfing habits.

He said on PBS television the NSA was simply collecting data on phone numbers and calls, which could then be accessed if they believed a suspect was involved in terrorism.

He said the schemes were subject to a system of "checks and balances" adding: "Congress is overseeing it, federal courts are overseeing it."

His speech will come nearly 50 years to the day after Kennedy's famed "Ich bin ein Berliner" address delivered from elsewhere in the city, two years after the erection of the Berlin Wall.

The Gate itself was the backdrop for another climactic moment in Cold War history, when Reagan famously beseeched then Soviet leader Mikhail Gorbachev to "tear down this wall" in 1987.

Unlike Reagan, who stood with the Wall behind him, Obama will face east when he gives his speech, in a symbol of the transformation of a city long divided by the Cold War.

Obama will hold bilateral talks and a press conference with Merkel, before the two leaders join for a ceremonial dinner. Obama will then fly back into the maelstrom of hostile US politics as he returns to Washington.