Sweden has deployed planes, ships and troops after suspicious underwater activity, possibly caused by submarines, in the waters of the Stockholm archipelago raised tensions.
Without revealing the nature of the threats, the Swedish government said it is not scaling down the security measures, despite not making any new detections overnight.
"This will continue until we consider that we are done. We are now trying to verify the information we received yesterday, which we assess that comes from trustworthy sources, and see whether it has any substance or not," Jesper Tengroth, press officer for the Swedish military, told the TT news agency.
About 200 troops are involved in the operation.
The Swedish government has not commented on the specifics of the operation, but said it trusts its military to do its job.
"We have support for the analysis work we are now carrying out in the area in the form of ships, planes and territorial army personnel. The area is heavily trafficked, which is why it's important for us to go public with information about what we're doing," a statement on the Swedish Armed Forces website read, citing a "credible source" for the information pertaining to the development.
It is widely believed the underwater activity has been caused by submarines.
"We gather intelligence with the sensors on the ships, on the helicopters and on land. We take this very seriously," Jonas Wikström, the commanding officer of the operation, told reporters.
Local reports have pointed fingers at Russian submarines for the incident, citing recent aerial confrontations between the Russian air force and US and Sweden's air forces.