The bags might be empty, but the threat isn’t.

A new FBI report is warning federal, state and local law-enforcement agencies around the United States about a new Islamic terror tactic: leaving suspicious, but harmless, bags and packages out in public.

The report says the unattended bags are designed to both sow fear in the public mind and to tie up precious bomb-squad resources.

A person who suggested the idea on a jihad-advocating Web site last month called for an invasion of “suspicious bags” in “the heart of Washington and New York,” ABC reported.

“The stated goal of the campaign was to exploit desensitization of first responders caused by response fatigue to suspicious, but harmless items,” the FBI internal report said.

“The poster suggested packing bags with innocuous items and placing them in public areas has the capability to occupy response assets and disrupt public infrastructure and transportation.”

Local law-enforcement sources told The Post they received the report from the FBI’s Washington field office.

Those sources also said there is no evidence that such a plot has been put into effect in New York City.

ABC noted that the tactic has been used in the past by anarchists in the United States, including during the 2000 Republican Convention in Philadelphia, in which knapsacks meant to be seen as bombs were left near convention sites.

Meanwhile, authorities in Yemen have arrested 12 Americans, possibly in connection to a joint US-Yemeni anti-terror campaign, a State Department spokesman said yesterday.

Yemeni officials last week had said they had detained Americans, Britons and an Australian as part of an investigation into increased activity by al Qaeda in that country.

Department spokesman P.J. Crowley, when asked whether the Americans were busted at the request of the US government, refused to answer directly.

“We are doing our best to help Yemen reduce the threat posed by al Qaeda in the Arabian Peninsula . . . Beyond that, I’m not going to talk about specifics,” Crowley said.

Yemeni security officials have said that some of the foreigners detained last week may be linked to a Nigerian man, Umar Farouk Abdulmutallab, who is accused of trying to blow up a Detroit-bound airliner last December.

(Source: NY Post)