WASHINGTON: The U.S. has suffered a spate of cyber-attacks, reportedly from overseas, successfully infiltrating key government systems including the White House and National Weather Service and prompting authorities to shut down the unclassified e-mail system at the State Department.

The systems at the White House, National Weather Service, U.S. Postal Service and the U.S. State Department have been hit by the cyber-attacks in recent weeks, officials said, adding FBI is now investigating at least four cyber-attacks that have infiltrated key government systems.

The attacks led the State Department to take the unprecedented step of shutting down its entire unclassified email system as technicians repaired damage from the cyber attack.

However, officials said, unlike in the past the attacks so far have hit only unclassified network.

The State Department, like any other large organisation that has a global span, is a constant target of cyber-attacks, its spokesman told reporters at a news conference.

"We closely monitor cybersecurity. We detected activity of concern several weeks ago. And as a result, we immediately formed a team to develop and implement a response plan, in coordination with cybersecurity experts from DHS and from other agencies," State Department Spokesman Jeff Rathke said.

"We are implementing carefully planned improvements to the security of our main unclassified network, taking advantage of a scheduled outage," Rathke said, adding that no classified systems have been affected by this incident so far.

"With regard to attribution, I don't have anything to share at this point on the origins of the intrusion. It's something that remains under investigation," he said.

Shawn Henry, who led the FBI's cyber division, and who now runs the security firm Crowdstrike, said the break-ins appear to be spy operations.

"This is most likely an intelligence collection operation. They are looking to gather intelligence about who the players are within the government, who they are communicating with and the new initiatives they are developing," Henry was quoted as saying by the CBS news.

Henry said that the spate of breaches might indicate that the U.S. businesses and agencies are getting better at detecting intrusions, but preventing them is a bigger challenge.

"I think our best way to get through this is to manage this, to be able to detect it quickly and then to mitigate it not unlike how we treat major diseases.

"We are not going to eradicate them. We have to detect them and we have to respond and mitigate the consequences," he was quoted as saying.

Meanwhile, the Pentagon said it has not been affected by the cyber-attacks and its line of communication remains secure.

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