Story highlights Vienna police warn European cities of possible terror threats

The possible attacks involve explosives or guns set for some time during the holidays

Vienna and other police in Europe have heightened security

(CNN) An unnamed "friendly" intelligence service has warned several European cities of possible terror attacks, according to a statement released Saturday by police in Vienna, Austria.

The attacks would involve explosives or guns and occur sometime between Christmas and New Year's Eve, according to the statement, which did not name the cities that have been warned.

The warning did include the names of several possible attackers the Vienna police have investigated without finding "concrete further results."

"Overall, this is a lead, which stipulates a higher than general abstract state of danger," the Vienna police said.

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In response to the terror threat warning, Vienna and other police in Europe have heightened the security alert by increasing police observation and surveillance at public venues, especially at key events and high-traffic areas.

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