US officials have obtained new intelligence deemed highly credible indicating al-Qaeda or other terrorists are in the United States and preparing to launch a major attack this northern summer, The Associated Press has learned.

The intelligence does not include a time, place or method of attack but is among the most disturbing received by the government since the attacks of September 11, 2001, according to a senior federal counter-terrorism official who spoke to the AP on condition of anonymity.

Of most concern, the official said, is that terrorists may possess and use a chemical, biological or radiological weapon that could cause much more damage and casualties than a conventional bomb.

"There is clearly a steady drumbeat of information that they are going to attack and hit us hard," said the official, who described the intelligence as highly credible.

The official declined to provide any specifics about the sources of the information but said there was an unusually high level of corroboration.

Despite that, the official said there was no immediate plan to raise the nation's terrorism threat level from yellow, or elevated, to orange, or high.

The threat level has been at yellow - midpoint on the five-colour scale - since January.

Attorney General John Ashcroft and FBI Director Robert Mueller plan a news conference tomorrow to outline an intensive effort by law enforcement, intelligence and homeland security officials to detect and disrupt any potential plots.

And the FBI plans to dispatch a bulletin to some 18,000 state and local law enforcement agencies warning of the threat.

Beginning with Saturday's dedication of the new World War II Memorial in Washington, the summer presents a number of high-profile targets in the United States.

They include the G-8 summit in Georgia next month that will attract top officials from some of America's closest allies, the Democratic National Convention in Boston in July and the Republican National Convention in August in New York.

The FBI and Homeland Security Department also are concerned about so-called soft targets such as shopping malls anywhere in the United States that offer a far less protected environment than a political convention hall.

US authorities repeatedly have said al-Qaeda is determined to mount an attack on US soil, in part to announce to the world that it remains capable of doing so despite the money and effort that has gone into homeland security in the wake of the September 11 attacks.

There also is concern terrorists might try to mount an attack to coincide with the November election.

The political fallout from the March 11 train bombings in Spain taught al-Qaeda that an attack timed to an election can have a major impact.

Spain's former ruling party was ousted in the voting that followed the bombing, which killed 191 and injured more than 2,000.

The official did not say how many suspected al-Qaeda or other terrorist operatives are believed in the country, whether they made their way into the United States recently or have been here for some time.

The FBI has warned in the past that Islamic extremist groups may attempt to recruit non-Middle Easterners or women for attacks because they would be less likely to arouse suspicion.

Special security attention already is being focused to the nation's rail, subway and bus lines.

The FBI last week sent out an intelligence bulletin to law enforcement agencies urging vigilance against suicide bombers, who have been used by terror groups worldwide to devastating effect but not so far in the United States.

Separately, Immigration and Customs Enforcement chief Michael Garcia told reporters today that some 2,300 of its agents are being deployed to assist in security for the high-profile events scheduled this summer in the United States.

These include as many as 20 agents each day working with the Secret Service to protect the campaigns of President Bush and Senator John Kerry, the Democratic presidential candidate.

Garcia said his agency also is working to "tighten the investigative system" to ensure that terrorists do not enter the United States by way of human smuggling operations or through the vast, largely unprotected border with Canada.