Police in Vienna have stepped up security around the city after a warning to several European capitals about a possible attack between Christmas and new year. They said that the warning, about a gun or bomb attack, did not specify any targets and that authorities in Vienna did not intend to cancel any specific events.

The warning did give several names of potential attackers, the statement said, but an investigation into the names had not produced concrete results.

Christoph Pölzl, a spokesman for Vienna 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. We have no evidence that they are in Vienna, and we have no evidence that they are even in Europe.” Pölzl refused to say which other European capitals had received the warning.

City authorities said they would increase surveillance of places where crowds are expected to gather, including transport hubs, and pay closer attention to suspicious bags or suitcases.

France remains on high alert after the Paris terror attacks in November. Last week the interior minister, Bernard Cazeneuve, instructed police and security forces to be particularly vigilant at festive church services.