TAIPEI (Reuters) - Taiwan is investigating an unprecedented case of threats made to five brokerages by an alleged cyber-group seeking payment to avert an attack that could crash their websites, an investigator and the securities regulator said on Monday.

Rick Wang, an official with Taiwan’s Financial Supervisory Commission (FSC), said each brokerage had received an email setting a deadline for the transfer of funds to avoid a distributed denial of service (DDoS) attack.

Such attacks, among the most common kind on the internet, overload a website until it is forced to inhibit access or go offline.

They have become common tools for cyber criminals trying to cripple businesses and organizations with significant online activities.

“We have never seen this on such a scale - five companies hit at one time with the same threat,” said Wang, adding that the regulator usually sees single instances of cyber-crime.

FireEye, a cybersecurity consultancy, said the attacks were similar to a wave of threatened denial of service attacks by a previously unidentified group that first appeared in Europe last month.

The Taiwan attacks do not pose a threat to the island’s broader trading and financial system, Wang said, but he added that the regulator had asked all securities firms to step up defensive measures.

One threat recipient, Masterlink Securities Corp, said its website had come under attack, but it had recovered and operations were normal.

“The emails were sent under the name of the ‘Armada Collective’,” said Chiu Shao-chou, an official of the internet cybercrime division of Taiwan’s Criminal Investigation Bureau, the government’s top investigation body.

The Armada Collective, a hacking extortion group, has been linked to financial blackmail heists elsewhere. But Chiu said the group has been put under watch and Taiwan investigators were still looking into the original source of the emails.

The email demanded payment in web-based digital currency bitcoin equivalent to about T$300,000 ($9,731.41), Taiwan media said.

None of the securities companies made any payments, Chiu said.

Another brokerage firm, Capital Securities Corp, was hit on Monday by a DDoS attack lasting 20 minutes before its system recovered, the regulator said, but it did not link the latest case to the threatening emails.

(This version of the story corrects sixth paragraph to show the attacks were similar to, not necessarily part of, a wave of attacks in Europe last month)