American authorities have arrested seven terror suspects in Miami in connection with the early stages of a plot to attack Chicago's Sears Tower, among other targets in the United States.

As well as the 110-floor Sears Tower -- the tallest building in the United States -- the group was planning to target Miami's FBI office, a U.S. government law-enforcement official told The Associated Press on condition of anonymity.

Sears Tower Managing Director Barbara Carley released a statement saying it would inappropriate to confirm or deny details of news reports about the building.

"However, Sears Tower security officials regularly speak with the FBI and local law enforcement authorities who track and investigate terrorism threats," she said.

"Today was no exception. Despite new information, law enforcement continues to tell us that they have never found evidence of a credible terrorism threat against Sears Tower that has gone beyond criminal discussions."

The official also told the wire agency that the alleged terror suspects are mainly Americans without any links to al Qaeda or other foreign organizations.

But a law enforcement source told Reuters the suspects had thought they were dealing with the international al Qaeda group but had been infiltrated by a U.S. government informant.

U.S. prosecutors in Miami confirmed they launched a series of raids Thursday night as part of ongoing investigations into a "terrorist-related matter."

As part of the raids, FBI agents swarmed a warehouse in Miami's Liberty City area, using a blowtorch to take off a metal door.

Terrorism expert Eric Margolis described Liberty City as a "no-go zone for outsiders."

It is "one of the most crime-ridden neighbourhoods in the Miami area," he said in a phone interview with CTV Newsnet.

A nearby resident told AP the suspects had been sleeping in the warehouse while running what appeared to be a "military boot camp.''

CNN reported that law enforcement sources described the suspects as members of a radical Muslim group.

Those unconfirmed reports appeared to be bolstered by neighbours' accounts to AP.

Residents living near the warehouse said the suspects were Muslims who said they had dedicated their lives to Allah and tried to recruit young people to join their group.

"They would come out late at night and exercise. It seemed like a military boot camp that they were working on there," Tashawn Rose told AP. "They would come out and stand guard."

Miami U.S. Attorney R. Alexander Acosta said in a statement the investigation was continuing and more details would be released Friday during a news conference.

"There is no imminent threat to Miami or any other area because of these operations,'' said Richard Kolko, spokesman for FBI headquarters in Washington.

CNN reported that the FBI operation was terrorism-related but that no weapons or explosives had been found.

"We do have an ongoing operation in Miami. We are conducting a number of arrests and searches and we'll have more about that when the operation is concluded -- probably tomorrow morning," FBI Director Robert Mueller said in an interview with CNN's Larry King.

"Because it's an ongoing operation, I really can't get into the details. But whenever we undertake an operation like this, we would not do it without the approval of a judge," he said.

Several terrorism investigations have been linked to South Florida. Many of the Sept. 11, 2001, hijackers, including ringleader Mohamed Atta, lived and trained in the area.

Jose Padilla, a former resident once accused of plotting to detonate a radioactive "dirty bomb" in the United States, is in Miami on charges he provided material support to terrorists and conspired to murder Americans.

With files from The Associated Press