Global banks are preparing to defend themselves against a potential North Korea hacking attack. Cybersecurity experts worry that North Korea will continue to embolden themselves as the threat of United State military action over the nuclear program looms.

The threat to banking institutions is very real. North Korean hackers have stolen hundreds of millions of dollars from banks during the past three years. A 2016 heist at Bangladesh Bank yielded $81 million, according to Dmitri Alperovitch, chief technology officer at cybersecurity firm CrowdStrike. “We know they attacked South Korean banks,” said security teams in the United States. They added that fears have grown that banks in the United States will be targeted next.

The North Korean government has repeatedly denied accusations of hacking by security researchers and several governments that it has carried out cyber attacks.  But Alperovitch told the Reuters Cyber Security Summit on Tuesday that banks were concerned Pyongyang’s hackers may become more destructive by using the same type of “wiper” viruses they deployed across South Korea and at Sony Corp’s Hollywood studio.

North Korean hackers could use what they have learned through previous cyber heists about financial networks gathered to disrupt banking operations, according to Alperovitch. He also said his firm has conducted “war game” exercises for several banks fearing a North Korean attack.

“The difference between theft and destruction is often a few keystrokes,” Alperovitch said.

John Carlin, a former U.S. assistant attorney general, told the Reuters summit that other firms, among them defense contractors, retailers, and social media companies, were also concerned. “They are thinking ‘Are we going to see an escalation in attacks from North Korea?’” said Carlin, chair of Morrison & Foerster international law firm’s global risk and crisis management team.

Some others say that it is highly unlikely North Korea will use cyber tactics to attack the US because the rogue regime fears retaliation. Jim Lewis, a cyber expert with Washington’s Center for Strategic and International Studies, is one of those persons. He said it is unlikely that North Korea would launch destructive attacks on American banks because of concerns about the reaction from the US.

The concerns appear valid when considering North Korea is defying the sanctions placed on them demanding they abandon their quest to obtain nuclear weapons.