The Czech secret service has blamed Russia for a series of bomb threats against schools in the Czech Republic and Slovakia, placing more pressure on already strained ties between Prague and Moscow.

Hundreds of Czech and Slovak schools were forced to close for several days in early September due to unprecedented bomb threats via email, according to local media. Nothing came of the threats and no evidence of explosives was found near the schools.

The head of the Czech Republic’s secret service, Michal Koudelka, warned the country’s parliament on Monday about cyber attacks against Czech entities.

“For example, the threatening emails in September about the placement of explosives targeting a number of schools in the Czech Republic and Slovakia, behind which there is also a clearly visible Russian trace,” Koudelka said.

“We are witnessing a kind of globalisation of evil, where the countries of the axis of evil — Russia, China, Iran and North Korea — support, complement and help each other achieve their goals. We are therefore witnessing a phenomenon that is very serious and dangerous,” he added. Related