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.

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.

Among the precautions, police will initiate more thorough security checks, ensure quick readiness in case of an emergency, and increase vigilance in terms of empty suitcases and bags, Vienna police said in the statement.

French National Police refused to comment on the warnings when contacted by CNN, but did say that more than 48,000 police officers are dedicated to security at sensitive sites during the school holidays from December 19 through January 4, and France plans to recruit 2,000 new police officers next month.

A Belgian prosecutor who serves as a government spokesman on terrorism issues also refused to comment when contacted by CNN.

CNN received no immediate responses to requests for comment from officials of European cities.

•Photo shows rescuers at work after the recent Paris terror attack in France.