Hackers believed to be working for Russia have been monitoring internal email traffic at the U.S. Treasury and Commerce departments, according to people familiar with the matter, adding they feared the hacks uncovered so far may be the tip of the icebergm, writes Reuters.

The hack is so serious it led to a National Security Council meeting at the White House on Saturday, said one of the people familiar with the matter.

U.S. officials have not said much publicly beyond the Commerce Department confirming there was a breach at one of its agencies and that they asked the Cybersecurity and Infrastructure Security Agency and the FBI to investigate.

National Security Council spokesman John Ullyot added that they “are taking all necessary steps to identify and remedy any possible issues related to this situation”.

The U.S. government has not publicly identified who might be behind the hacking, but three of the people familiar with the investigation said Russia is currently believed to be responsible for the attack. Two of the people said that the breaches are connected to a broad campaign that also involved the recently disclosed hack on FireEye, a major U.S. cybersecurity company with government and commercial contracts.