Al-Qaeda could attempt a large-scale attack in the United States to coincide with the presidential elections, Homeland Security Secretary Tom Ridge has warned.
Democrats used the latest warning, which provided no specific details, to urge the Republican-led Senate to act on legislation for homeland security funding.
Mr Ridge said the warning was based on intelligence received from credible sources gathered over the past months.
"Since September 11th, 2001, we have had intelligence that al-Qaeda intends to launch more attacks against the homeland," he told a news conference.
"Credible reporting now indicates that al Qaeda is moving forward with its plans to carry out a large-scale attack in the United States in an effort to disrupt our democratic process."
But he said authorities had no specific intelligence about whether al-Qaeda was targeting either the Democratic convention later this month in Boston or the Republican convention in New York at the end of August.
The terror alert level will stay at elevated, or yellow - the third of five levels on the colour-coded scale.
Mr Ridge said there were "permanent protections in place today" that did not exist a year ago.
When asked about the timing of his announcement - in a week Democrats have captured attention with the announcement of presidential candidate Mr John Kerry's running mate, Mr Ridge and the White House denied any political motivation.
Osama bin Laden, believed to be hiding along the Afghanistan-Pakistan border, is directing an al-Qaeda effort to launch an attack in the United States sometime this year, the New York Timesreported on Friday, citing US officials.