Vienna police said Saturday they are increasing security as a precaution after a warning to several European capitals about the possibility of an attack using explosives or firearms between Christmas and New Year, but they see no reason to call off any of the planned events.

Vienna police said in a statement that the tip came from an unidentified “friendly” intelligence service came in days before Christmas. Authorities were also given names of several potential attacks, but further examination of the list and other investigations brought no “concrete results” so far.

Christoph Poelzi, a spokesman for Vienna police, told the Austria Press Agency there was no concrete threat to a specific place at a particular time.

“We do not know if these people exist in real life, or if they are only names with no real person behind them. We have no evidence that they are in Vienna, and we have no evidence that they are even in Europe,” he added, according to the Guardian.

Authorities are expected to increase surveillance around the city and increase patrols in areas where the volume of people is expected to be higher including transport hubs, and keep an extra eye on possible suspicious bags or suitcases.

Last week, France’s interior minister Bernard Cazenueve, instructed police to be extra vigilant during prayer services throughout the country. France remains on high alert stemming from the Paris terrorist attacks in November.

The Associated Press contributed to this report.