US authorities hunt pipe bombs suspects

US authorities are hunting for suspects in the sending of package bombs to high-profile Democrats in what has been called a terrorist act that has raised tensions in a nation already polarised ahead of elections in less than two weeks.

At least eight suspicious parcels were intercepted before reaching any intended recipients, who included former US President Barack Obama, former Secretary of State and presidential candidate Hillary Clinton and Obama's attorney-general Eric Holder.

Former CIA Director John Brennan, prominent Democratic Party donor George Soros and California Congresswoman Maxine Waters, a an outspoken critic of President Donald Trump, were also targets of packages.

All of the targets are frequently maligned by right-wing critics.

The bomb threats aggravated tension ahead of November 6 elections that will decide whether Democrats take control of one or both houses of Congress from Republicans and deny Trump the majority his party now holds in both.

None of the eight packages detonated, and nobody was injured. There was no immediate claim of responsibility.

An FBI-led Joint Terrorism Task Force of federal, state and local law enforcement agencies "will continue to work to identity and arrest whosoever is responsible for sending these packages", FBI Director Christopher Wray said in a statement.

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

The CNN bureau in New York 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 neighbourhood near Central Park.

The package contained an envelope of white powder that experts were analysing, Police Commissioner James O'Neill said.

Some leading Democrats were quick to accuse Trump himself of stoking the potential for political violence by frequently engaging in hyper-partisan, vitriolic rhetoric.

Trump told a political rally in Wisconsin on Wednesday 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."

He also said the media had a responsibility "to stop the endless hostility and constant negative and oftentimes false attacks and stories".

Democratic party leaders in Congress said Trump's call for unity rang hollow because of his past statements condoning acts of violence.

Last week, Trump, who has joined other Republicans in accusing Democrats of encouraging "mob" tactics, heaped praise on a Montana congressman who assaulted a reporter in 2017.

At an event in Texas on Wednesday, Brennan faulted Trump for frequent inflammatory rhetoric.