On the second anniversary of the Sept. 11, 2001 terror attacks, the State Department on Thursday issued a last-minute worldwide warning of more possibly "devastating" Al Qaeda (search) actions.

"We are seeing increasing indications that Al Qaeda is preparing to strike U.S. interests abroad," the State Department said in the warning.

"We expect Al Qaeda will strive for new attacks that will be more devastating than the September 11 attack, possibly involving nonconventional weapons such as chemical or biological agents," the advisory added. "We also cannot rule out the potential for Al Qaeda to attempt a second catastrophic attack within the U.S."

The Department of Homeland Security, however, said there were no plans to raise the nation's terrorism threat alert level (search) from yellow, which signifies an "elevated" risk of attacks. The five-level, color-coded system was last raised to orange, or "high risk," for 11 days in May.

In the past year, Al Qaeda and connected organizations have carried out bombings in Riyadh, Saudi Arabia, Casablanca, Morocco, and Bali, Indonesia.

"European or Eurasian locations could be venues for the next round of attacks, possibly to closely coincide" with the Sept. 11 anniversary, the State Department said.

Department officials confirmed to Fox News that at the end of working hours Wednesday, there had been no plans to issue an advisory, but the situation changed very suddenly overnight.

"The U.S. government remains deeply concerned about the security of U.S. citizens overseas," the warning said. "U.S. citizens are cautioned to maintain a high level of vigilance, to remain alert and to take appropriate steps to increase their security awareness."

The latest caution updated a July 29 warning about possible hijackings of commercial aircraft by terrorists.

Homeland Security officials said there were no known domestic threats relating to Thursday's warning.

"We know of no specific or credible threat, although we are always aware of the threat posed by Al Qaeda to U.S. persons and interests around the world," an FBI official told Fox News on Thursday.

Homeland Security Secretary Tom Ridge said Thursday that there was no need to raise the alert level despite the new, threat-filled videotape of Usama bin Laden (search) aired on Arabic satellite station Al Jazeera on Wednesday.

Ridge said "yellow" reflects "a high level of risk" and that the bin Laden tape didn't change that.

"We know that America is [Al Qaeda's] primary target," Ridge said in a television interview. "And they know that every single day we're doing everything we can to not only prevent their attacks but reduce our own vulnerability in this country to their carnage and their death and their destruction and their hate."

The threat level was raised to the second-highest level — orange — for the first anniversary of Sept. 11.

In a recent interview, Fox News' Mike Emanuel asked Ridge if there was any way to calculate how many terror attacks had been prevented in the past two years.

"Enhanced security, improved security measures in certain places, have at least caused [Al Qaeda] to pull back from operations they had planned on," Ridge said.

"That doesn't say that they may not go back and revisit it, but we have learned from many, many sources that a more alert, a more prepared, a more secure America with visible signs of that preparation and security has been a deterrent," Ridge added. "I can't tell you how many times — we just know it has been."

Ridge said that although terror threats, specifically from Al Qaeda, still remained, state and municipal governments were more prepared than they were two years ago. He noted that the intelligence community meets by video conference twice a day to discuss threats and determine appropriate action.

White House press secretary Scott McClellan said Thursday that state and local officials and the private sector should take appropriate security measures under the current threat level. The federal government sent out an advisory last week with a reminder of those steps.

"There are still those who are enemies of peace and freedom that seek to harm us," McClellan said.

Evan Kohlmann, senior terrorism expert with the Investigative Project, noted that mid-level Al Qaeda commanders had taken the places of senior leaders who had been killed or arrested. There had also been an increased amount of "chatter" from Al Qaeda in recent months, he added.

On Thursday, CIA officials verified with reasonable certainty that one of the voices heard on the latest Usama bin Laden videotape, broadcast Wednesday by Qatar-based satellite news channel al-Jazeera, was that of Al Qaeda's day-to-day leader, Ayman al-Zawahri.

Another voice purporting to be that of bin Laden himself also provides commentary to the video footage, which shows bin Laden and al-Zawahri hiking down a rocky slope, but the CIA could not determine the speaker's identity.

"[The videotape indicates] to us that Al Qaeda is up to something," said Kohlmann. "There may be attacks planned and the State Department warnings may be prophetic."

But State Department spokesman Richard Boucher said Thursday that the new tape was not the "precipitating factor" that led to the last-minute caution.

"The worldwide caution was based on first of all information that we have obtained through yesterday ... Sept. 11 being a moment that we all want to be especially careful and where people might want to try to repeat some things they've done in the past," Boucher said.

"So it was a combination of things, information analysis and the anniversary. I don't think the tape was a precipitating factor in any way. It was more the kind of information that had accumulated and the kind of analysis that was being done."

Thursday's warning said that terrorist actions might include, but would not be limited to, suicide operations, hijackings, bombings or kidnappings. Commercial aircraft and threats to include conventional weapons, such as explosive devices, might also be involved.

"Terrorists do not distinguish between official and civilian targets," it said.

These targets may include facilities where American citizens and other foreigners congregate or visit, including residential areas, clubs, restaurants, places of worship, schools, hotels, outdoor recreation events or resorts and beaches.

"U.S. citizens should remain in a heightened state of personal security awareness when attendance at such locations is unavoidable," the warning said.

Fox News' Mike Emanuel, Anna Stolley, Teri Schultz and The Associated Press contributed to this report.