Dozens of journalists who cover the U.S. Defence Department vacated their offices in the Pentagon and returned their credentials on Wednesday as new restrictions on press access took effect.

The Defence Department had set a Tuesday deadline for news outlets to either sign a new Pentagon access policy or lose access to press credentials and Pentagon workspaces.

At least 30 news organisations, including Reuters, declined to sign the new policy, citing a threat to press freedoms and their ability to conduct independent newsgathering on the world's most powerful military.

The policy requires journalists to acknowledge new rules on press access, including that they could be branded security risks and have their Pentagon press badges revoked if they ask department employees to disclose classified and some types of unclassified information.

The Pentagon Press Association, which represents more than 100 news organisations, including Reuters, said in a statement that Wednesday was "a dark day for press freedom that raises concerns about a weakening U.S. commitment to transparency in governance, to public accountability at the Pentagon and to free speech for all."

Chief Pentagon spokesperson Sean Parnell said in a statement on Monday: "The policy does not ask for them to agree, just to acknowledge that they understand what our policy is. This has caused reporters to have a full blown meltdown, crying victim online. We stand by our policy because it's what's best for our troops and the national security of this country."

The Pentagon declined to make additional comment on Wednesday.

Journalists described the press area at the Pentagon on Wednesday as unusually quiet, as they removed furniture, computer servers, TV studio soundproofing material and other contents.

“I’ve never seen that place not buzzing like a beehive,” said JJ Green, National Security Correspondent at Washington news radio station WTOP.

Green, who has worked as a national security correspondent for 20 years, turned in his press credential Wednesday morning. Television outlets have until Friday to remove their gear.

Credentialed reporters have traditionally been limited to unclassified spaces in the Pentagon and have worked across the hallway from the Pentagon press office, which has allowed them access to department spokespeople. Press badges signify that they have gone through a background check.

"We’ve never been allowed to just bolt right on into classified areas or people's offices," said Stephen Losey, a reporter who covers the Air Force for Defence News.

“I don't know anybody who would purposely eavesdrop or anything like that, which is what some people have made it seem like we're doing.”

Some journalists interviewed by Reuters said the new restrictions won't keep them from reporting on the U.S. military.

"The irony of irony is that Pentagon reporters are not having conversations about controlled information in the hallways,” said a member of the Pentagon Press Association speaking on condition of anonymity. “We’re doing it over (the encrypted app) Signal."

The Pentagon's new policy is the latest expansion of restrictions on press access under Defence Secretary Pete Hegseth, a former Fox News host. Fox News is among the news organisations that have refused to sign on to the new press restrictions.