The president-elect’s remarks came as it emerged that the hackers accessed systems at the National Nuclear Security Administration, which maintains the country’s stockpile of nuclear weapons
Mr Biden says his transition team have been briefed on the hack, which took place over a number of months at a string of federal agencies.
“There’s a lot we don’t yet know, but what we do know is a matter of great concern,” said Mr Biden in a statement.
“I have instructed my team to learn as much as we can about this breach, and vice president-elect Harris and I are grateful to the career public servants who have briefed our team on their findings, and who are working around-the-clock to respond to this attack.”
Donald Trump has not yet made a statement on the Kremlin-linked hack but Mr Biden said that he would make cybersecurity “a top priority at every level of government.”
“A good defense isn’t enough; we need to disrupt and deter our adversaries from undertaking significant cyber attacks in the first place,” he added.
“We will do that by, among other things, imposing substantial costs on those responsible for such malicious attacks, including in coordination with our allies and partners.
“Our adversaries should know that, as President, I will not stand idly by in the face of cyber assaults on our nation.”
The government’s Cybersecurity and Infrastructure Security Agency believes the hack first began in March and that the hackers “demonstrated patience, operational security, and complex tradecraft in these intrusions.”