WASHINGTON, July 25 (Xinhua) -- The U.S. Transportation Security Administration (TSA) has alerted law enforcement officers across the country to be on the lookout for what could be "dry runs" for a terrorist attacks at airports.

    The TSA sent an unclassified advisory on July 20 to law enforcement agencies, after series of suspicious incidents occurred at U.S. airports, raising the possibility that recent activity could be "pre-attack security probes," U.S. media reported Wednesday.

    The advisory, which has been widely reported, detailed four incidents from the past 11 months in which screeners found unusual objects with items such as wires, switches, tubes, cell phone components that could mimic bomb components in passengers' checked or carry-on bags.

    "The unusual nature and increase in number of these improvised items raise concern and TSA personnel should continue vigilance for groupings of ordinary items that look like IED components," the alert said.

    The bulletin warned that terrorists could be conducting repeated operations to desensitize security officials.

    The TSA, however, downplayed on Wednesday the significance of the advisory.

    "This bulletin is not classified nor does it contain secret material," the TSA said in a statement.

    The TSA said the advisory was one of more than 90 unclassified bulletins sent to police during the past six months to provide information to front line officers.

    "There is no intelligence that indicates a specific or credible threat to the homeland," the TSA said.

    The United States has tightened security at its airports since the Sept. 11 attacks, by manning airports with additional security personnel and installing high-tech devices to screen both passengers and their baggage.