WASHINGTON/NEW YORK - Former US President Barack Obama and former Secretary of State Hillary Clinton were among the targets of suspected package bombs delivered to several high-profile Democrats and CNN in what New York officials on Wednesday called an act of terrorism.

At least eight suspicious parcels were intercepted before reaching any intended recipients, including Obama's attorney general, Eric Holder, former CIA Director John Brennan and prominent Democratic Party donor George Soros.

Two packages were sent to California Congresswoman Maxine Waters, the FBI said.

Investigators were trying to track down a suspicious package they believe was addressed to Obama's vice president, Joe Biden, a federal official told Reuters late on Wednesday.

None of the eight packages detonated, and nobody was hurt.

There was no immediate claim of responsibility.

But news of the threats heightened tensions in a nation deeply polarized ahead of elections on Nov. 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.

President Donald Trump told a political rally in Wisconsin his government would conduct "an aggressive investigation."

"Any acts or threats of political violence are an attack on our democracy itself," Trump said. "We want all sides to come together in peace and harmony."

But he said the media has a responsibility "to stop the endless hostility and constant negative and oftentimes false attacks and stories."

MANILA ENVELOPES

The first package, which turned up on Monday, was addressed to Soros, the billionaire financier and frequent target of right-wing conspiracy theories.

The parcel intended for Holder ended up rerouted to the return address printed on all five packages - the Florida office of US Representative Debbie Wasserman Schultz, who formerly chaired the Democratic National Committee, according to the Federal Bureau of Investigation.

The FBI said late on Wednesday it had confirmed that two additional packages, similar in appearance to the others, were addressed to US Representative Waters, a veteran Democratic congresswoman from Los Angeles and also an outspoken Trump critic.

An earlier FBI statement said the other packages consisted of a manila envelope with a bubble-wrap interior containing "potentially destructive devices." Each bore a computer-printed address label and six "Forever" postage stamps.

Other officials said the devices contained in the envelopes were all similar to one found in the mailbox of the Soros home and later detonated by police. At least one bomb was packed with shards of glass, one federal source said.

The US Secret Service intercepted packages addressed to Obama at his Washington home and to Clinton, the 2016 Democratic nominee for president, at her home in New York state.

The CNN bureau in New York also received a package addressed to Brennan, who has appeared as an on-air CNN analyst, leading police to evacuate the Time Warner building in a busy Manhattan neighborhood near Central Park.

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. Obama and Clinton were not at risk, they said.

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

"So far the devices have been what appeared to be pipe bombs," John Miller, the New York City police deputy commissioner for intelligence and counterterrorism, told a news conference.

"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.