A group of hackers believed to be working for the Russian government gained access to unclassified White House computer networks over the past couple of weeks, interrupting some services. Cybersecurity teams are investigating.

White House officials said that the hackers haven't harmed any systems and that they don't think the White House classified network has been breached, according to a report from the Washington Post.

"In the course of assessing recent threats, we identified activity of concern on the unclassified Executive Office of the President network," said one White House official. "We took immediate measures to evaluate and mitigate the activity. ... Unfortunately, some of that resulted in the disruption of regular services to users. But people were on it and are dealing with it."

Officials have declined to comment on who specifically was behind the attack, but reports from security firms indicate that the hackers are Russian and likely are working for the government. The FBI, National Security Agency and Secret Service are all taking part in the investigation.

"Certainly a variety of actors find our networks to be attractive targets and seek access to sensitive information," the White House official said. "We are still assessing the activity of concern."

Other targets of the Russian hackers include NATO, the Ukrainian government and U.S. defense contractors. The U.S. considers Russia to be in the top tier of states with cyber-capabilities.

"It looks to be criminal and of Russian origin," the U.S. official said, according to the Wall Street Journal.

When it comes to gauging whether that criminal element is working with the government, however, "you're back into that gray area. You really can't tell," the official said.

U.S. concerns about Russian spying have increased as relations between the two countries have disintegrated after Russia's conflict with Ukraine. Officials said that even if Russian cyber-spying has increased, it's still difficult to detect.