Terrorists may be conducting “dry runs” at US airports to test security before a possible attack, according to a Transportation Security Administration warning to airport screeners.

In at least four incidents over the past year, security screeners have found items in carry-on luggage — blocks of cheese taped to electrical components, for example — that resembled homemade bombs, according to the TSA's July 20 memo.

“Past terrorist attacks and plots show that such testing generally indicates attacks will soon follow,” said the internal memo, which was posted on the internet by NBC News on Tuesday. None of the passengers in question has been linked with criminal or terrorist organisations so far, the memo said.

The TSA described its unclassified warning as one of 90 released this year and said it had no specific information of a pending attack.