Initial signs suggested the breach was long-running and significant, the people familiar with the matter said. The Russian hackers, known by the nicknames APT29 or Cozy Bear, are part of that nation's foreign intelligence service and breached email systems in some cases, said the people familiar with the intrusions, who spoke on the condition of anonymity because of the sensitivity of the matter.

The same Russian group hacked the State Department and the White House email servers during the Obama administration.

All of the organisations were breached through the update server of a network management system called SolarWinds, according to four people familiar with the matter.

The company said Sunday in a statement that monitoring products it released in March and June of this year may have been surreptitiously weaponized with in a "highly-sophisticated, targeted...attack by a nation state."

The scale of the Russian espionage operation is potentially vast and appears to be large, said several individuals familiar with the matter.

 "This is looking very, very bad," said one person. SolarWinds products are used by more than 300,000 organisations across the world. They include all five branches of the U.S. military, the Pentagon, State Department, Justice Department, NASA, the Executive Office of the President and the National Security Agency, the world's top electronic spy agency, according to the firm's website. SolarWinds is also used by the top 10 U.S. telecommunications companies.

Apparently, APT29 compromised the SolarWinds server that sends updates so that any time a customer checks in to request an update, the Russians could hitch a ride on that update to get into a victim's system.

Reuters described the breach as "so serious it led to a National Security Council meeting at the White House" although quite what the point there was. Donald Trump thinks Russian hackers are all fake news made up by the liberal press to discredit his good friend Vladimir Putin.