US Secretary of State Mike Pompeo said Russia was likely behind the massive cyberattack on multiple government agencies and organisations.
"We can say pretty clearly that it was the Russians that engaged in this activity," Pompeo said on a conservative talk radio show, stating that they were still "unpacking" what it was.
He called it a "significant effort" to use software to embed code inside US government systems, but did not provide other details.
US President Donald Trump has been silent on the hack which officials have said poses a "grave risk" to the country.
The US Cybersecurity and Infrastructure Security Agency said this was "a patient, well-resourced, and focused adversary that has sustained long duration activity on victim networks."
What do we know about the cyber attack?
It was revealed last week that attackers had installed malware onto a US firm SolarWinds' monitoring platform. The platform could have been installed by up to 18,000 customers, SolarWinds said.
This widespread hack impacted multiple US government agencies, organisations and private companies and had been going on since at least March.
It was first revealed to the government by the cybersecurity company FireEye.
FireEye CEO Kevin Mandia wrote in a December 8 blog post that his company had been subject to a hack and that he had "concluded we are witnessing an attack by a nation with top-tier offensive capabilities."
Microsoft's president Brad Smith, whose company is assisting in the response, said that the hack had affected at least 40 government agencies and other organisations.
At least 80% of these organisations were in the United States but there were victims in seven other countries: Canada, Mexico, Belgium, Spain, the United Kingdom, Israel and the UAE.
Experts told the Associated Press that rebuilding the networks and discovering what happened could take months.
Democrats also say Russia likely behind it
Democratic members of Congress who were briefed on the hack also said that the Russians were likely behind it.
"We have been learning over the last few weeks about a massive cybersecurity breach of our government, probably by the Russians," Illinois Senator Dick Durbin said on the Senate floor.
"It has all the earmarks and fingerprints of a Vladimir Putin project, an ongoing effort by Russia to undermine the United States, to compromise our national security and to create chaos whenever possible."
Durbin said that the information they had was "unnerving" and that there were more government agencies discovering they had been compromised every day.
"How much they know, what they've gained, how much they've compromised us, we don't know yet," Durbin said.
President-elect Joe Biden said in a statement on Thursday that what had been revealed about the hack was "of great concern".
Biden said he would make cybersecurity a "top priority" and deal with the breach from the moment he takes office.