Vienna police
Vienna police have learnt about suspects who could carry out attacksDominic Ebenbichler/Reuters

Security has been stepped up in several European cities following a "friendly" intelligence service alert regarding suspected terrorists who could launch attacks between Christmas and New Year. According to the warning, terrorists could use explosives or guns to carry out the attacks.

"In the lead-up to Christmas, a friendly intelligence service sent a warning to numerous European capitals saying potential bomb or gun attacks could take place in areas with large crowds between Christmas and New Year," the Vienna police said in a statement. The tip-off contains several names of potential attackers. The police, however, have not divulged which cities were under threat.

"Overall, this is a lead, which stipulates a higher than general abstract state of danger," the police said. "We do not know if these people exist in real life or if they are only names with no real person behind them," Christoph Pölzl, a spokesman for the Vienna police, told the Guardian.

"We have no evidence they are in Vienna, and we have no evidence they are even in Europe," he added. Investigations into the names have so far not yielded any concrete results, the police said.

After the warning was issued, Vienna police and other forces in Europe have increased surveillance at public places. The Vienna police, however, said that upcoming events in the city would go ahead as planned.

Following the 13 November Paris attacks in which 130 people were killed, several European cities have heightened security. France has deployed 48,000 police officers to avert potential threats during the holiday season. Last week, French interior minister Bernard Cazeneuve told police and security forces to be vigilant, especially during church services.