The US military has blocked access to the Guardian’s website for troops in the Middle East and south Asia, after disclosures about widespread US surveillance.

On Friday, the Pentagon and the US army told the Guardian that automated content filters installed on Department of Defense (DoD) networks to prevent the unauthorized dissemination of classified information had blocked access to selected aspects of the Guardian’s website.

But in for troops in Afghanistan, the Middle East and south Asia, the restriction applies to the entire website.

“This is a theater-wide block,” reads a page that loads when troops in Afghanistan using the Defense Department’s non-classified internet protocol (NIPR) network attempt to access the Guardian online.

“There are many reasons why this site might be blocked. It may be blocked for your protection, the protection of DoD assets or blocked based on Usfor-A [US forces command-Afghanistan] information systems security policy enclosure 18, Centcom regulation 25-206, joint ethics regulation (JER) 5500.7 or DAA directives,” the routed site reads.

Usfor-A is the US component to the Nato command in Afghanistan known as Isaf. It is unclear if the block on the Guardian’s website applies to non-American personnel in Afghanistan, but if they use the DoD’s networks to get online, non-American servicemembers would not be able to access the Guardian website either.

But the block does not come from Isaf. It comes from US central command, the command responsible for US military operations in the Middle East and south Asia.

“US central command is among other DOD organizations that routinely take preventative measures to safeguard the chance of spillage of classified information on to unclassified computer networks, even if the source of the information is itself unclassified,” said US army Lt Col Steve Wollman, a spokesman for central command. “One of the purposes for preventing this spillage is to protect Centcom personnel from inadvertently amplifying disclosed but classified information.

“Additionally, classified information is not automatically declassified simply because of unauthorized disclosure,” Wollman continued.

“Classified information is prohibited from specific unclassified networks, even if the information has already been published in unclassified media that are available to the general public, such as online news organizations.”

Wollman confirmed that the block applies not only to troops in Afghanistan, but for those deployed anywhere in central command’s area of operations, which includes the Middle East, south Asia, and the command's headquarters in Florida.

The US military’s online filters for classified information are not new. In 2010, the air force’s protectors of “network hygiene” blocked access to the websites of news organizations that published classified material acquired by radical transparency group WikiLeaks, including the New York Times, the Guardian, El Pais, Le Monde and Der Spiegel.

But not every news website that published classified material detailing the breadth of National Security Agency surveillance is blocked on military networks in Afghanistan. The Washington Post website, which hosts some of the same classified information published by the Guardian -– as well as classified slides about the Prism internet-content collection program that the Guardian has not published – is unrestricted.

It is unclear how long central command’s ban on the Guardian website will last. Wollman did not respond to a question about why the Washington Post’s website is accessible.

• This article was amended on 2 July 2013 to remove a reference to Wikileaks "purloining" classified information.