BERLIN - Germany sent hundreds of police officers into railway stations, airports and other public places Wednesday, after the nation's top security official warned of an increased threat from Islamic extremists.

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The move came after new, tangible intelligence came to light over the past weeks and months, Interior Minister Thomas de Maiziere said. Germans will notice a visible response to the threat, including an increased police presence at airports and railway stations to remain in place until further notice, he said.

"These measures are aimed at prevention and deterrence," de Maiziere said. "We are showing force, that we will not be bullied."

Despite the heightened security presence, de Maiziere urged calm. Officials did not provide specific details about the threat.

"There is reason for worry, but there is no reason for hysteria," he said. "We will not allow international terror to limit us in our way of life and our culture of freedom."

Germany has no color-coded or numbered-tier system for measuring security levels, comparable to those in the US, Britain or France and tends to be far more cautious in publicly discussing the degrees of threat, on grounds it could endanger efforts to prevent an attack.

Germany has more than 4,900 soldiers serving in northern Afghanistan as part of the NATO security force there and has long expressed concern it could be targeted by extremists.

Chancellor Angela Merkel's spokesman, Steffen Seibert, said there were no plans to cancel or change any trips or public appearances by the German leader. He echoed de Maiziere's statement that the country must not allow terror threats to impinge on its freedom  "then we would give the terrorists a cheap victory."

Though Germany has escaped any major terrorist attacks like the Madrid train bombings of 2004 and the London transit attacks of 2005, at least two major plots have been thwarted or failed before they could be carried out.

"Besides the previously known findings, we now have additional, relevant indications of possible danger that security authorities agree justify our current assessment that we are now facing a new situation," de Maiziere said.

He cited a tip from another unspecified country about a suspected attack planned for the end of November though he didn't say where it was supposed to take place.

That tip arrived after the interception of two mail bombs mailed from Yemen to the US - one of which went through a German airport before it was found in Britain. The Yemen plot points to "the adaptability and the persistence of terrorists in pursuing their aims," and underlines "the reliability of some leads," de Maiziere said in a statement to reporters, without elaborating.

He said German authorities had also gathered concrete intelligence of their own pointing to "sustained efforts" by Islamic extremist groups to plan attacks in Germany. Consequently, authorities have launched investigations that could lead to charges.

Maj Lauralee Flannery, a spokeswoman for the US military's European Command, based in Stuttgart, said there had not been any specific threats against military bases and while forces remained vigilant, "We're not doing anything different."

Germany did not elevate its security situation last month when the United States warned its citizens of possible attacks in Europe, triggering both Great Britain and France to increase their alert levels.

US Embassy spokesman Mitchell Moss said that he could not comment on specific security measures at American diplomatic facilities in Germany, but said that the security level had not changed recently.

"We haven't changed our security posture based on Minister de Maiziere changing the threat level in Germany," he said.

De Maiziere said the current risk assessment was comparable to the one before last year's national election - when the country also raised security. One trigger for that alert was a video in which a German speaker threatened a "rude awakening" if Germans did not push their political parties to withdraw the country's soldiers from Afghanistan.

By midday Wednesday, police carrying automatic weapons and wearing protective gear could be seen patrolling railway platforms at the Berlin's Central Station, as well as stations in Duesseldorf, Munich and Frankfurt. But Germans did not appear to be particularly concerned.

"I don't feel threatened, otherwise we would not be walking around here in Berlin," said Angelika Balkenhoel, arriving from nearby Hamburg.