Counter terrorism officials have issued security bulletins to police around the US about terrorist interest in attacking stadiums, entertainment complexes and hotels, the latest in a flurry of such internal warnings as investigators chase a possible bomb plot in Denver and New York.

The two bulletins, sent to police departments on Monday and obtained by The Associated Press, say they know of no specific plots against such sites, but urged law enforcement and private companies to be vigilant. These two bulletins followed hard on the heels of a similar warning about the vulnerabilities of mass transit systems.

The bulletin on stadiums notes an Al-Qaida training manual specifically lists "blasting and destroying the places of amusement, immorality, and sin... and attacking vital economic centres" as desired targets of the organisation.

"While DHS and FBI have no information regarding the timing, location or target of any planned attack, we believe it is prudent to remind transit authorities to remain vigilant," Homeland Security spokesman Sean Smith said on Monday night.

Separately, law enforcement officials said a Colorado man may have been planning with others to detonate backpack bombs on New York City trains in a terrorism plot similar to past attacks on London's and Madrid's mass-transit systems.

The investigation and the earlier warning about mass transit system have already prompted officials around the nation to step up patrols.