US President Barack Obama makes a dinner speech at the Chralottenburg Castle in Berlin June 19, 2013. [Photo/Agencies]

BERLIN - President Barack Obama defended US anti-terrorism tactics on a visit to Berlin on Wednesday, telling wary Germans Washington was not spying on the emails of ordinary citizens and promising to step up efforts to close the Guantanamo Bay prison.

On the 50th anniversary of John F. Kennedy's famous "Ich bin ein Berliner" speech, Obama made his first presidential visit to the German capital, a favoured destination of US leaders during the Cold War.

He held talks with Chancellor Angela Merkel and gave a speech at the Brandenburg Gate in which he unveiled a proposal for new talks with Russia on slashing nuclear arms arsenals.

Obama, who attracted a crowd of 200,000 adoring fans when he last passed through in 2008 during his first campaign for the presidency, remains popular in Germany.

But revelations before the trip of a covert US Internet surveillance programme, code-named Prism, caused outrage in a country where memories of the eavesdropping East German Stasi secret police are still fresh.

Merkel said at a joint news conference that also touched on Afghanistan, Syria and the global economy, that the two leaders had held "long and intensive" talks on the spying issue, noting that some questions still needed to be cleared up.

Obama tried to reassure his host, who as a pastor's daughter growing up in the communist East experienced the Stasi first-hand.

"This is not a situation in which we are rifling through the ordinary emails of German citizens or American citizens or French citizens or anybody else," Obama said.

"This is not a situation where we simply go into the Internet and start searching any way we want. This is a circumscribed system directed at us being able to protect our people and all of it is done under the oversight of the courts."

In a message which seemed designed for her domestic audience, Merkel told Obama that balance was essential in government monitoring of Internet communications.

"I made clear that although we do see the need for gathering information, the topic of proportionality is always an important one and the free democratic order is based on people feeling safe," said the 58-year-old chancellor.

Obama countered that the US had thwarted at least 50 threats because of its monitoring program, including planned attacks in Germany.

"So lives have been saved and the encroachment on privacy has been strictly limited," he said.

A poll last week showed 82 percent of Germans approve of Obama, but the magic of 2008, when he was feted like a rock star, has faded amid concerns about his anti-terrorist tactics.