The pipe bombs, at least five in all, were intercepted before any reached their intended recipients, who included former Secretary of State Hillary Clinton, former attorney-general Eric Holder and former CIA director John Brennan, the FBI said.

None of the packages detonated and nobody was hurt. There was no immediate claim of responsibility.

But news of the threats heightened tensions in a nation deeply polarised before elections on November 6 that will decide whether Democrats take control of one or both houses of Congress from Republicans and deny President Donald Trump the majority his party now holds in both.

The first package, which turned up on Monday, was addressed to billionaire financier George Soros, a prominent Democratic Party donor and frequent target of right-wing conspiracy theories.

But the parcel intended for Holder ended up rerouted to the return address that had been printed on all five packages – the Florida office of US Representative Debbie Wasserman Schultz, who formerly chaired the Democratic National Committee, according to the FBI.

US Representative Maxine Waters, a veteran Democratic congresswoman from Los Angeles, said her Capitol Hill office was the target of a suspicious package that “has been referred to the FBI”.

The five other packages confirmed by the FBI were described as similar in appearance, consisting of a manila envelope with a bubble-wrap interior containing “potentially destructive devices”.

Other officials said the devices contained in the envelopes were similar to one found in the mailbox of the Soros home and later detonated by police.

The US Secret Service intercepted packages addressed to Obama at his Washington home and to Clinton at her home in New York state.

The CNN bureau in New York also received a package, leading police to evacuate the Time Warner building in Manhattan.

New York City Police Commissioner James O’Neill (on the left), New York State Governor Andrew Cuomo (on the right), and New York City Mayor Bill de Blasio (center, speaking) speaking about the bomb threat at CNN’s New York City headquarters. Photo: Michael Brochstein/Sipa USA

“This clearly is an act of terror attempting to undermine our free press and leaders of this country through acts of violence,” New York Mayor Bill de Blasio told a news conference.

He dismissed social media postings suggesting the packages were a liberal hoax, telling CNN, “This is the real thing.”

‘Trump and his press secretary Sarah Huckabee Sanders condemned the acts as “despicable” and vowed to bring those responsible to justice.

Trump, who usually relishes in making partisan attacks on Democrats, also issued a call for unity against acts or threats of political violence.

All the targets are frequently maligned by right-wing critics. Trump has repeated criticised CNN as “fake news” and disparaged the mainstream news media as an “enemy of the people”.

“There is a total and complete lack of understanding at the White House about the seriousness of their continued attacks on the media,” CNN president Jeff Zucker said in a statement.

The package to Clinton was found late on Tuesday while the one addressed to Obama was recovered early on Wednesday, both during routine off-site mail screenings, the Secret Service said.

Officials said the package received by CNN was addressed to Trump critic Brennan, the former CIA chief who appeared on CNN before joining rival MSNBC as a paid contributor.

The package sent to CNN also contained an envelope of white powder that experts were analysing, Police Commissioner James O’Neill said.

– Reuters