WASHINGTON (AP) Federal authorities expressed increased alarm Thursday about an intrusion into U.S. and other computer systems around the globe that officials suspect was carried out by Russian hackers. The nations cybersecurity agency warned of a grave risk to government and private networks.

The Cybersecurity and Infrastructure Security Agency said in its most detailed comments yet that the intrusion had compromised federal agencies as well as critical infrastructure in a sophisticated attack that was hard to detect and will be difficult to undo.

CISA did not say which agencies or infrastructure had been breached or what information taken in an attack that it previously said appeared to have begun in March.

This threat actor has demonstrated sophistication and complex tradecraft in these intrusions, the agency said in its unusual alert. CISA expects that removing the threat actor from compromised environments will be highly complex and challenging.

In a statement, the Department of Energy confirmed the agency was affected, but that the breach was isolated to business networks only and not national security functions.

The hack, if authorities can indeed prove it was carried out by a nation such as Russia as experts believe, creates a fresh foreign policy problem for President Donald Trump in his final days in office.

Trump, whose administration has been criticized for eliminating a White House cybersecurity adviser and downplaying Russian interference in the 2016 presidential election, has made no public statements about the breach.

President-elect Joe Biden said he would make cybersecurity a top priority of his administration, but that stronger defenses are not enough.

We need to disrupt and deter our adversaries from undertaking significant cyberattacks in the first place, he said. 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.