The latest cities to suffer mass evacuations include Yekaterinburg and Rostov-on-Don, with Moscow being targeted for the fourth day in a row on Tuesday, the RIA Novosti news agency reports.

An unnamed Interior Ministry source told RBC that an international hacking group could be behind the bomb threats that have plagued Moscow and a dozen provincial cities.

A second unidentified source who took part in an inter-agency meeting between the ministry and the Federal Security Services (FSB) on the anonymous calls, confirmed the theory.

“These calls were identified as calls made from Brussels,” the Interior Ministry source told RBC.

The individual who took part in an inter-agency meeting said the ministry and the FSB are developing software to defend against the calls.

They also said the suspected hackers could in the future demand a ransom to stop the bomb scares.

RBC estimated on Monday that the mass evacuations may have cost the authorities around 300 million rubles ($5.2 million) so far.