Guests of the Kosmos Hotel wait outside after being evacuated from the building in Moscow on September 13, 2017. (AFP PHOTO)

MOSCOW, Sept. 14 (Xinhua) -- Russian President Vladimir Putin has been informed of the ongoing wave of bomb threat phone calls that plagued across the country, which can only be perceived as "telephone terrorism," the Kremlin said Thursday.

"Undoubtedly, it is telephone terrorism, telephone hooliganism," Kremlin spokesman Dmitry Peskov was quoted as saying by RIA-Novosti.

"The relevant authorities are taking all the necessary measures in order to identify those responsible for this series of calls. We are waiting for the results of this work," he added.

A wave of anonymous calls about planted bombs in various administrative and commercial buildings continue in Russia for the fifth day in a row, although the number is declining.

Tass news agency reported that 48 threats were received in four Russian cities on Thursday.

So far, over 130,000 people have been evacuated throughout Russia from more than 420 facilities, according to RIA-Novosti. All calls proved to be hoaxes.

Earlier on Thursday, a law enforcement source told Sputnik news agency that the anonymous calls were initiated by persons abroad who were linked with the Islamic States terrorist group, according to data obtained as a result of a series of search and investigation.

Tass quoted a separate source in the law enforcement as saying that it is established that the series of bomb threat calls was "a well-orchestrated campaign" organized by a certain group of people or even one person, and that checks are being made if some extremists groups were involved.