It is great to see the priority that the new administration is giving to cyber.
The leading candidate for Cyber Director is Jen Easterly, a former high ranking National Security Agency official, according to four people familiar with the selection process.
"The nature and extent of information that has been potentially accessed is still being determined, but it may include some commercially and personally sensitive information," he added.
"It's unfortunate that it has taken over three weeks after the revelation of an intrusion this significant for this Administration to finally issue a tentative attribution," said Senator Mark Warner, vice chairman of the Senate Intelligence committee.
Top US officials - including Secretary of State Mike Pompeo - have blamed Russia for the espionage operation, although some officials and experts have told Reuters it is too soon to know for sure who is behind the breach.
O'Brien "is holding NSC meetings tonight and tomorrow morning and will convene a high-level interagency meeting this week," National Security Council spokesman John Ullyot said.
Because the attackers could use SolarWinds to get inside a network and then create a new backdoor, merely disconnecting the network management program is not enough to boot the hackers out, experts said.
US media reports said the FBI was investigating a group working for the Russian foreign intelligence service, SVR, and that breaches had been taking place for months.