Flight attendants in the United States are calling for the government to step up action against human trafficking by funding mandatory training to help them spot victims in the air.

The Association of Flight Attendants-CWA (AFA), the world's largest flight attendant union, said its 50,000 members from 18 airlines can act as a massive network of trained eyes in the skies, potentially saving millions of lives.

The call came two years after U.S. authorities launched an anti-trafficking campaign called Blue Lightning to encourage airlines to train personnel on how to identify potential traffickers and report suspicious activity to police.

"As aviation's first responders, we are charged with the safety, health and security of the passengers in our care," AFA International President Sara Nelson said in a statement on Friday.

"Traffickers steal lives. But for a window of time, we can see it, report it and law enforcement can bring justice.

Trafficking victims are "hidden in plane site", the union said on its website.

The U.S. government has estimated that between 14,500 and 17,500 people are trafficked into the United States each year for forced labour and the sex industry, in addition to thousands more transported around the country to work in slavery.

Globally it is estimated about 36 million people are living in slavery generating about US$150 billion a year in profits for their exploiters.

The U.S. government agencies responsible for transportation have a computer-based training module and printed material available, while the union offers its members tips on how to recognize victims of trafficking.

But AFA wants to go further and is calling on Congress to make the training program mandatory for the aviation industry and to allocate government money to finance this.

It did not provide a cost estimate and was not immediately available for further comment.

The U.S. Customs and Border Protection, which leads the government's Blue Lightning programme, was not immediately available for comment.