Al-Qaeda is aiming to attack the US to try to undermine the presidential election in November, Homeland Security Secretary Tom Ridge has warned.
Ridge's department constantly reviews reported threats
There were credible reports that the terror group was moving ahead with plans to "disrupt our democratic process," Mr Ridge told reporters.
The intelligence agencies were actively working on finding out exactly where, when and how an attack might happen.
But the US would not be raising its level of terror alert, Mr Ridge said.
Mr Ridge told a news conference in Washington that information was pointing to al-Qaeda plans for a large-scale attack in the coming months.
Security was being tightened ahead of the Democratic Party's national convention later this month, and the Republicans' convention in August, he said.
The US presidential election is to be held on 2 November.
Mr Ridge conceded that they did not have precise knowledge about the time, place and method of attack.
"But along with the CIA, FBI and other agencies, we are actively working to gain that knowledge," he said.
The BBC's Gordon Corera in Washington says the lack of specific information has led some Americans to question the usefulness of this and previous similar warnings.
Democrats have also raised concerns that the warnings themselves could be used by the administration to have an impact on the election.
Mr Ridge said the colour-coded warning system would remain at yellow, which indicates a "significant" threat of a terrorist attack.
He also said security across the country was constantly being strengthened, citing a number of measures that included:
- Full nationwide connectivity to the Homeland Security Operations Center, allowing information to be received in real time
- new pilot programme to track high-risk trucks; operation to track rail shipments
- deployment of handheld radiological detectors for use by law enforcement officials
- monitoring via web-enabled perimeter cameras of chemical facilities of greatest concern.