Hackers broke into systems used by top US Treasury officials during a massive cyberattack on government agencies and may have stolen essential encryption keys, a senior lawmaker said Monday. 

Senator Ron Wyden, who sits on both the Senate Intelligence and Finance Committees, said after a closed-door briefing that the hack at the US Treasury Department "appears to be significant."

Dozens of email accounts were compromised, he said in a statement.

"Additionally the hackers broke into systems in the Departmental Offices division of Treasury, home to the department's highest-ranking officials," said Wyden.

"Treasury still does not know all of the actions taken by hackers, or precisely what information was stolen."

The US government admitted last week that computer systems in multiple departments were penetrated by attackers who hacked in through widely used security software made by the US company SolarWinds.

So far officials have said the hackers broke into computers at the State Department, Commerce Department, Treasury, Homeland Security Department, and the National Institutes of Health.

But experts have said they fear far more of the government could be affected, including US intelligence bodies, given the ubiquitousness of the SolarWinds security software.

Members of Congress briefed by US intelligence, as well as Secretary of State Mike Pompeo and Attorney General Bill Barr, have all said Russians were behind the hack. Trump has downplayed Russian role in the attack.

Republican US Senator Mitt Romney said a widespread data breach across the US government was "extraordinarily damaging" and that President Donald Trump has a "blind spot" when it comes to Russia.