A major anti-terror operation is underway in the US after suspicious packages were sent to prominent opponents of President Donald Trump.

Explosive devices have been sent to Barack Obama, Hillary Clinton, and several prominent politicians in New York, Florida and Washington DC.

A live device and white powder have also been sent to former CIA Director John Brennan at CNN in New York.

The BBC's Nick Bryant told Mike Hosking the packages could have been detonated if authorities hadn't intercepted them.

"They're saying they are very crude and very simple but they are also functional and that means they could have dine damage if they got through the screening."

The Secret Service said the package addressed to the Chappaqua, New York, home of Clinton and former president Bill Clinton was recovered late Tuesday. The package sent to Obama's Washington home was intercepted early Wednesday, authorities said.

Both packages were intercepted by Secret Service personnel working at off-site facilities near their homes in New York and in Washington, according to a person familiar with their work. All mail and packages addressed to former presidents and their immediate family are pre-sorted and screened by Secret Service personnel.

An Obama representative referred questions to the Secret Service. Clinton spokesman Nick Merrill said the package was "not in proximity to their residence" and also deferred other questions to the agency.

CNN's headquarters at the Time Warner Center in New York were also evacuated Wednesday morning due to a suspicious package found there. The network broadcast footage of its staffers outside on the Manhattan streets below, where anchor Jim Sciutto could be seen on a cellphone reporting that he was told a suspicious package was mailed to the CNN building.

Jeff Zucker, president of CNN, wrote in a letter to employees that the center was "evacuated out of an abundance of caution" after the package was found in the mailroom. The New York police said its officers were "investigating a suspicious package in Columbus Circle," which is where the Time Warner Center is located.

Officers at a checkpoint near the home of President Barack Obama. Photo / AP

The devices sent to Clinton and Obama are suspected to be the work of the same person who sent a similar device to Soros earlier this week, according to two law enforcement officials who spoke on the condition of anonymity to discuss an ongoing investigation. Soros, an 88-year-old holocaust survivor, has funnelled much of his fortune into liberal projects around the world, making him a frequent target of rhetoric on the right.

Based on the timing and the material, law enforcement officials suspect the same person is behind all three devices, and officials are scrambling to determine if any other such devices were sent to anyone.

The New York police said they were investigating the suspicious package at CNN and working with federal partners involved in the investigations into what was sent to Obama, Clinton, Soros and the news network, according to a police official. As part of that, New York police were increasing patrols at areas linked to those three people and inspecting packages sent to locations tied to them. The police were also increasing patrols at high-profile areas, including media locations, in New York City.

A woman walks her dogs past a checkpoint near the home of President Barack Obama. Photo / AP

The White House and other figures across Washington decried the explosive devices.

"We condemn the attempted violent attacks recently made against President Obama, President Clinton, Secretary Clinton, and other public figures," Sarah Sanders, the White House press secretary, said in a statement Wednesday morning. "These terrorising acts are despicable, and anyone responsible will be held accountable to the fullest extent of the law. The United States Secret Service and other law enforcement agencies are investigating and will take all appropriate actions to protect anyone threatened by these cowards."

Suspicious letters and packages have been sent to the president's children, including Donald Trump Jr., the president's eldest son, and Eric Trump. On Wednesday, Trump Jr. tweeted: "As someone whose family has directly been the victim of these mail threats I condemn whoever did this regardless of party or ideology. This crap has to stop and I hope they end up in jail for a long time."

A letter was sent to Trump Jr.'s home earlier this year that resulted in his wife going to the hospital. A man later pleaded guilty to sending threatening letters with white powder to Trump's sons and other public figures.