(NewsNation Now) — Cyberattacks against Ukrainian government websites and affiliated organizations have bombarded the country as the Russian invasion of the Eastern European nation continues.

The websites of Ukraine’s defense, foreign and interior ministries were hit this week with crippling attacks. Many of the same sites were similarly knocked offline earlier this month in attacks that the U.S. and U.K. governments quickly blamed on Russia’s GRU military intelligence agency.

Now, U.S. leaders have an urgent message for Americans: shields up! Cyber experts told NewsNation that as U.S. sanctions against Russia increase, cyber warfare against the U.S. will increase, too.

Most of the online assaults were denial-of-service attacks — where sites are flooded with so many users they crash.

“Denial of service attacks are pretty easy for folks to do, and pretty hard to defend against,” Security expert Kevin Bong said.

The Department of Homeland Security and the White House say there is no specific threat to the United States at the moment, but the threat is real.

“If they think they have nothing to lose, the gloves will come off, and they will try to inflict the maximum amount of pain on us,” says computer security industry expert Dmitri Alperovitch.

Hackers could shut down banks, hospitals, transportation, power grids and businesses, even taking down money-wiring systems. This could cause disastrous issues for the stock market.

“Imagine if your company is trying to do payroll and deposit money into bank accounts, but they can’t get the money for the payroll because they have not gotten wires from their customers,” Alperovitch said.

They could cause disastrous issues for the stock market.

Cyber experts told NewsNation that small- to medium-sized businesses that do not have cyberattack mitigation plans are the most at risk for threats.

“What we need private sector companies to do is to harden their cyber security capabilities now, now is the time to do it,” White House press secretary Jen Psaki

DHS says make sure your software and anti-virus systems are up to date, take steps to quickly notice an intrusion on your software, have a crisis plan and always have a backup away from network connections.

Cyberattacks have been a key tool of Russian aggression in Ukraine since before 2014, when the Kremlin annexed Crimea and hackers tried to thwart elections. They were also used against Estonia in 2007 and Georgia in 2008. The West blames Russia for some of the most damaging cyberattacks on record, which caused more than $10 billion of damage globally.

Social media has long vexed Putin’s attempts to control the information coming out of Russia, and Friday his government announced the “partial restriction” of access to Facebook after the social media network banned the accounts of several Kremlin-backed media over Moscow’s invasion of Ukraine.

Russian state communications watchdog Roskomnadzor said Friday it demanded that Facebook lift the bans it placed Thursday on state news agency RIA Novosti, state TV channel Zvezda and pro-Kremlin news sites Lenta.Ru and Gazeta.Ru. The agency said Facebook didn’t reinstate the media outlets.

Roskomnadzor said the “partial restriction” takes effect Friday, without clarifying what exactly the move means.In its official statement, Roskomnadzor cast its action as ’“measures to protect Russian media.”