:: If you are about to embark on a journey to the US and you see “SSSS” code on your boarding pass, then be prepared for a potentially lengthy experience at security.

The code is part of the ongoing crackdown on terrorism, which stands for “Secondary Security Screening Selection,” that has been introduced for people traveling to the US.

The heightened security will likely mean extra bag searches, more pat downs and identification checks in the departures terminal.

People who find the code on their passes are said to be picked at random, but according to British press reports some tourists have complained that they are stopped almost every time they have traveled stateside since the scheme began.

Many people have taken to Twitter with the hashtag “#SSSS” to vent their frustration.

“Once again I’m being asked for my passport to get on a domestic flight. Wth @tsa?! I had to stand at TSA again to be SSSS’d for 35mins.” Wrote Twitter user @hellochaosX3 who appears to be subjected to the issue on a regular basis.

According to the UK daily The Sun it has been suggested that people who buy tickets using cash, or those who booking one-way trips will be most likely to be targeted by the “SSSS” system.

But security officials the move is necessary to help make air travel safer as they single out potential threats trying to enter the US.

Seth Weinstein, a USA customs attorney, said the Terrorist Screening Database maintained by the Terrorist Screening Center to determine who qualifies for “SSSS.”

“It’s quite difficult to get off the SSSS list, but anyone who wants to can appeal against the decision,” Weinstein said