A man who had a long-running feud with an Annapolis newspaper blasted his way through its newsroom with a shotgun on Thursday, killing at least five people in one of the deadliest attacks recorded on a US media outlet, authorities said.

The suspect, described as a white man in his 30s who lives in Maryland, fired through a glass door, looked for victims and then sprayed the newsroom of the Capital Gazette newspaper group in Annapolis with gunfire, police and a witness said.

Among the victims was Capital Gazette editor Rob Hiaasen, according the Baltimore Sun, which owns the newspaper group.

The suspect is Jarrod Ramos, 38, of Laurel, the Capital Gazette and Baltimore Sun reported citing law enforcement.

In 2012, Ramos brought a defamation lawsuit against Eric Hartley, formerly a staff writer and columnist with The Capital, and Thomas Marquardt, then editor and publisher of The Capital, according to a court filing.

In 2015, Maryland’s second-highest court upheld a ruling in favour of the Capital Gazette and a former reporter who were accused by Ramos of defamation, the Baltimore Sun reported.

The newsroom looked "like a war zone," Phil Davis, a Capital Gazette crime reporter who was in the building at the time of the shooting, said in an interview with the Baltimore Sun.

He said he was hiding under his desk along with other newspaper employees when the shooter stopped firing, the Capital Gazette reported on its website.

"I don't know why he stopped," said Davis, who later said he was safe. "But as much as I'm going to try to articulate how traumatising it is to be hiding under your desk, you don't know until you're there and you feel helpless."

When police found the suspect, his weapon was on the ground and "not in his immediate proximity," Steve Schuh, Anne Arundel county executive, told cable news station CNN. Police said they recovered what they thought might have been an explosive device.

One law enforcement source told CBS News the suspect had no identification on him. Two law enforcement sources told CBS News the suspect used a shotgun.

FBI assisting investigations

The suspect appears to have damaged his fingertips to try to avoid detection and is refusing to cooperate with law enforcement, Baltimore TV station WJZ and other local media reported.

The Federal Bureau of Investigation is on the scene assisting local authorities, the official said.

Capital Gazette, owned by the Baltimore Sun, runs multiple newspapers out of its Annapolis office and the group includes one of the oldest newspapers in the United States - the Gazette, which traces its origins back to 1727.

Capital Gazette publishes a stable of newspapers in and around Annapolis, home of the US Naval Academy. The papers have thrived by focusing on local news in the shadows of two much larger competitors, the Washington Post and Baltimore Sun.

Law enforcement in Baltimore and New York City deployed extra officers to the office of the New York Times and other major media outlets as a precaution, authorities said.

"There is no active threat at this time," said Officer Andrew Lava, a New York police spokesperson.

The shooting drew the attention of media groups, including Reporters Without Borders, which said it was deeply disturbed by the events that unfolded in Annapolis.

White House condemns attack

US President Donald Trump has been briefed on the shooting, White House spokesperson Lindsay Walters said.

"My thoughts and prayers are with the victims and their families. Thank you to all of the First Responders who are currently on the scene," Trump said in a tweet.

White House Press Secretary Sarah Sanders said on Twitter: "A violent attack on innocent journalists doing their job is an attack on every American."

As a presidential candidate and after his election, Trump has repeatedly criticised the press, referring to several major news organizations as "fake news" and calling a group that included the New York Times, CNN and CBS "the enemy of the American people."

Live video images showed people leaving the building, walking through a parking lot with their hands in the air. Scores of police vehicles were on the scene. About 170 people were evacuated.

Jimmy DeButts, an editor at the Capital Gazette, tweeted that he was devastated, heartbroken and numb.

"I'm in no position to speak, just know @capgaznews reporters & editors give all they have every day. There are no 40 hour weeks, no big paydays - just a passion for telling stories from our community," he wrote. 

One of the group's flagship papers, the Capital, plans to publish a Friday edition, several reporters with the group said.

"I can tell you this: We are putting out a damn paper tomorrow," reporter Chase Cook wrote on Twitter.