Vienna police have stepped up armed police patrols across the city, including churches, after internal intelligence received information suggesting a possible Islamist attack.

"How long this increased protection of the sites will last cannot be estimated at this time," police wrote on Twitter.

From there, they emphasize that if the threat is reduced in a certain location, a new announcement will be issued to the public as a matter of urgency.

The police warned citizens about the increased number of armed police patrols.

"You will notice that there are currently more police forces patrolling the city with special equipment. The reason is a non-specific threat against churches. We will continue to keep you informed," the police added on Twitter.

The Vienna police said that the Austrian internal intelligence service has information that an attack with an Islamist background is being planned in the capital and that preventive measures are common in such cases.