HACK ATTACK: State Department computers were compromised by intruders thought to be working from inside Russia

The U.S. State Department, already beset with allegations that it hasn't kept a tight enough leash on its officials' email accounts, shut down some of its unclassified email servers on Friday in order to clear out what sources say was a rash of malware inserted by foreign hackers.

Cybersecurity officials at State believe Russian hackers are to blame but described the breach of their networks only as 'activity of concern.'

The State Department did its best to handle the problem quietly, making no announcement during the regular daily reporters' briefing on Friday and issuing a subdued statement 21 minutes before the end of the work day.

ABC News reported that the cyber intrusion is one of the most serious in the agency's history.

But State framed it as a 'planned outage' and a routine 'upgrade' to its digital security systems, blocking its employees from sending outside emails and shuttering their Web browsers for an unspecified stretch of time that could last for days.

'As a part of the Department of State's ongoing effort to ensure the integrity of our unclassified networks against cyber attacks,' spokeswoman Jen Psaki said in a statement, 'the Department is implementing improvements to the security of its main unclassified network during a short, planned outage of some internet-linked systems.'

'The Department continues to closely monitor and respond to activity of concern on our unclassified network. Such activity is something we take very seriously.'

NOTHING TO SEE HERE -- MOVE ALONG ... State Department spokeswoman Jen Psaki downplayed the damage, saying that a 'planned outage' was in the works

MALWARE: Hackers inserted malicious code into nonclassified computers at the State Department, leaving tech experts no choice but to shut everything down and clean it up

'There has been no compromise of any of the Department's classified systems, nor of our core financial, consular, and human resource systems,' Psaki claimed.

'The recent increase in news reports regarding cyber incidents reflects that the Department is among a growing list of public institutions and private industries facing an increasing number of sophisticated cyber threats.'

This isn't the first such outage to hit America's diplomatic nerve center: The same kind of event happened once in November when the State Department carried out what it called an 'upgrade' following an October cyber attack.

Two State Department employees told Daily Mail Online that they couldn't reach websites from their office computers during the dinner hour on Friday, and confirmed that no emails were going in or out other than interdepartmental messages.

Hackers have run roughshod over computer servers in the executive branch of the U.S. government in recent years, targeting the White House on a regular basis.

In October, following an outage covering much of the Executive Office of the President, White House Press Secretary Josh Earnest told reporters it should ' not be particularly surprising' that 'there are many people around the world who would love to gain greater insight into the activities of the United States government by collecting information from the White House network.'

Many did find it surprising, however, to hear Earnest acknowledge that the computer infrastructure of 1600 Pennsylvania Avenue is 'subject to daily cyberattacks, or at least efforts to infiltrate it.'

The FBI, the NSA and the Secret Service were all involved in an investigation, with insiders telling news organizations that – as with Friday's State Department trouble – Russian hackers were the most likely suspects.

In August 2014 a cybersecurity firm discovered that Russian hackers had stolen 1.2 billion user names and passwords in a series of Internet heists affecting 420,000 websites.

And a months-long sustained cyber attack on JP Morgan Chase last summer also originated in Russia, security officials believe, and may have also targeted as many as nine other banks.

COULD IT BE? Russia's government is still angry at the Obama administration for imposing tough eceonimc sanctions on the nation's oligarchs following President Vladimir Putin's annexation of Crimea

At least 83 million bank accounts were affected, mostly belonging to consumers but includnig about 7 million commercial customers as well.

Hackers got access to customers' names, addresses, phone numbers and email addresses.

Saparately, the FBI offered a $3 million reward last month for information leading to the capture of a Russian hacker believed to be behind the electronic theft of $100 million from American bank accounts.

Evgeniy Bogachev is accused of deploying a 'botnet' – a hijacked super-network of personal computers controlled remotely without the owners' knowledge – by spreading malicious software through spam emails.