WASHINGTON (Reuters) - The U.S. State Department on Monday said its unclassified email systems were the victim of a cyber attack in recent weeks, around the same time as White House systems were breached, but no classified data was compromised.

The department shut down portions of its unclassified system over the weekend to improve its security but said they should be back online shortly, though it declined to predict when.

As a result of the shutdown, State Department employees are unable to access the Internet through their unclassified system or to reliably receive e-mails from people outside the agency. Unclassified e-mails within the department continue to flow.

"The State Department, like any other large organization that has a global span, is a constant target of cyber attacks," State Department spokesman Jeff Rathke told reporters. "We detected activity of concern several weeks ago."

Rathke said the agency's classified systems were not breached, adding: "We have no reason to believe classified information was compromised."

The spokesman also said the State Department breach was part of the same incident recently reported by the White House's Executive Office of the President.

While the department detected the infiltration of its system some weeks ago, it did not close down the unclassified system until over the weekend, during a previously scheduled shutdown.

The State Department breach follows similar intrusions disclosed in recent months at the White House, the Office of Personnel Management and, just last week, the U.S. Postal Service and National Oceanic and Atmospheric Administration.

USPS said the personal information of more than 800,000 employees may have been compromised, as well as data on customers who contacted its call center during the first eight months of the year.

At NOAA, four of the agency's websites were affected.

The State Department has agreed to brief lawmakers on the cyber attack, according to U.S. Representative Elijah Cummings.

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In a letter on Monday to Secretary of State John Kerry, Cummings, the senior Democrat on the House government reform committee, also asked State to provide more information by Jan. 5 to help Congress as it considers cybersecurity laws and other ways to protect consumer and government information.

A Pentagon spokesman said on Monday none of the military's systems has been affected.

(Additional reporting by Doina Chiacu and David Brunnstrom; Writing by Doina Chiacu and Arshad Mohammad; Editing by Jim Loney and Dan Grebler)