Airport security officers have been told to look out for terrorists practising carrying explosive components on to aircraft.

The alert follows four suspicious seizures at airports since last September.

The unclassified alert was distributed on July 20 by the Transportation Security Administration (TSA) to government air marshals, its own transportation security officers and other law enforcement agencies.

The seizures at airports in San Diego, Milwaukee, Houston and Baltimore included "wires, switches, pipes or tubes, cell phone components and dense clay-like substances", including block cheese, the bulletin said.

"The unusual nature and increase in number of these improvised items raise concern," it said.

Security officers were urged to keep an eye out for "ordinary items that look like improvised explosive device components".

The 13-paragraph bulletin was posted on the internet by NBC Nightly News, which first reported the story, but TSA spokeswoman Ellen Howe said: "There is no credible, specific threat here."

Homeland Security spokesman Russ Knocke described the notice as the latest copy of a routine information bulletin with TSA workers, airport employees and law enforcement officials.

The bulletin said a joint FBI-Homeland Security Department assessment found that terrorists had conducted probes, dry runs and dress rehearsals in advance of previous attacks.

It cited various types of rehearsals conducted by terrorists before the September 11 attacks on the World Trade Centre and the Pentagon as well as the July 7 London bombings.

The bulletin said initial investigation had not linked the passengers carrying the suspicious items with terrorist organisations, but it added that most of their explanations for carrying the items were suspicious.