Russian authorities have evacuated dozens of buildings in Moscow and other cities this week after a flurry of anonymous calls claiming that bombs were planted there.

The official Tass news agency, citing an unnamed security-service source, said more than 50,000 people had been evacuated in the capital after about 100 threats.

All of the threats have so far turned out to be false alarms.

On Wednesday, three of Moscow’s main train stations were checked, along with the city’s famous GUM department store near Red Square, and more than 20 other buildings, according to RIA Novosti.

Citing an unnamed source in the emergency services, the state-owned news agency reported that the hoaxes had so far affected more than 20 cities, and that many of the calls were believed to have been made from Ukraine.

Relations between Moscow and Kiev are at a low after Russia annexed Crimea from Ukraine in 2014 and pro-Russian separatists in eastern Ukraine rebelled against Kiev’s rule.