The US Attorney General, John Ashcroft, has designated 46 organisations as terrorist groups and banned their members from entering the United States.
Mr Ashcroft also announced the formation of a new anti-terrorist tracking task force. He said the authorities will also crack down on suspects already in the country.
Earlier, the United States tightened security around major nuclear facilities following a new warning about the threat of further terrorist attacks.
The Federal Aviation Authority has placed a week-long ban on any aircraft movements within 20 kilometres of more than 80 nuclear plants.
The only exception to the restrictions are planes used by the emergency services, which must have approval from air traffic control.
The measures were announced within 24 hours of the government warning of further terrorist attacks.
