U.S. on watch ahead of bin Laden anniversary

(CBS/AP) Federal authorities are urging local law enforcement to be vigilant with the one-year anniversary of Osama bin Laden's death just days away, but said there is "no indication of a credible threat or plots against the U.S." linked to the May 2 date.

The FBI, Department of Homeland Security and U.S. Northern Command sent a joint advisory to local and state law enforcement late Wednesday as a reminder that al Qaeda and its affiliates still aspire to attack the U.S.

A law enforcement source told CBS News that there has been an increase of messages to coincide with the first anniversary of bin Laden's death, posted on three or four known Jihadist extremist web forums calling for followers to consider all forms of an attack against transportation, churches and public venues such as theaters and sporting arenas in the United States including unconventional attacks such as chemical or biological.

Although the forums are explicit in aspirations there is absolutely no indication that an attack is planned, the source told CBS News investigative producer Pat Milton. The web site messages are also calling for electronic jihad, low level cyber attacks, such as denial of services, the source told Milton.

In addition, the source told CBS News, there is a "continuous stream of jihadist rhetoric" encouraging followers to duplicate the active shooter style attack in France, in which military and civilian personnel were killed earlier this month. That shooter was believed to have visited a violent jihadist forum before his fatal shootings.

Radical jihadist web sites continue to advocate attacks on American targets and U.S. officials say emerging al Qaeda affiliates like the Yemen-based al Qaeda in the Arabian Peninsula and the Somalia-based al Shabaab may present the greatest danger.

Special Section: The killing of Osama bin Laden

Al Qaeda in the Arabian Peninsula, or AQAP, has been a persistent concern since 2009, when one of its adherents nearly brought down a jetliner over Detroit on Christmas. In the past six months, counterterrorism officials have seen what they consider an increase in intelligence about potential threats from the group, an intelligence official speaking on condition of anonymity to discuss sensitive information told the Associated Press.

Officials are worried that the terror group "intends to advance plots along multiple fronts, including renewed efforts to target Western aviation," according to a joint intelligence bulletin circulated Wednesday from U.S. Northern Command, the FBI and Homeland Security Department.

The terror group has twice tried to attack U.S.-bound flights and is considered the most active al Qaeda affiliate, recruiting Westerners.

Other al Qaeda affiliates, including al-Shabaab in Somalia, have pledged to avenge bin Laden's death. But intelligence officials have not seen signs of current plots against the U.S. Still, officials urged law enforcement to be on the watch.

"We remain concerned that terrorists not yet identified by the intelligence community and law enforcement could seek to advance or execute attacks with little or no warning on or about the anniversary of bin Laden's death," the intelligence bulletin said.

Authorities frequently issue similar advisories ahead of notable events and anniversaries.

The heightened sense of security was also seen overseas, with the the U.S. Embassy in Pakistan releasing a fresh security warning to Americans in that country.

The embassy said on its website Thursday that its employees would be restricted from restaurants and markets in the capital, Islamabad, for the next two weeks.

It didn't say why, but the period covers the first anniversary of the U.S. raid that killed Osama bin Laden as well.

Militants have been known to stage attacks on or close to the anniversary of significant events.

U.S. diplomats already operate under tight security in Pakistan, which is home to an array of violent extremists.