US intel agencies warn of an ongoing cyber attack on government

US intelligence agencies have warned of a "significant" cyber attack on several federal departments uncovered over the weekend remains ongoing.

"This is a developing situation, and while we continue to work to understand the full extent of this campaign, we know this compromise has affected networks within the federal government," the FBI, the director of national intelligence and the Cybersecurity and Infrastructure Security Agency (CISA) said in a joint statement late Wednesday.

The content the hackers sought to steal -- and how successful they were -- remains unknown at this time.

The government, meanwhile, has rushed to assess the extent of the breach.

The March attack on software created by Texas-based IT company SolarWinds -- in which hackers installed malware -- continued for months until it was discovered by cybersecurity company FireEye.

Hackers reportedly breached software used by the US Treasury Department and the Commerce Department, allowing them to view internal email traffic. 

US Secretary of State Mike Pompeo also pointed to Moscow on Monday, saying the Russian government had made repeated attempts to breach US government networks.

The FBI said it has opened an investigation to identify and pursue those responsible for the hack.

After the attack was detected, CISA ordered federal agencies to power down the breached software.

The agencies have created a coordination unit and emergency talks are being held at the White House on a daily basis to discuss the government's response.