Today's trafficking in women and children for commercial sex work and forced labour is worse than the historic African slave trade, a senior Vatican official said Tuesday.

At a news conference in Vatican City, Cardinal Renato Martino, a former Vatican envoy to the United Nations, said modern-day forms of slavery are an increasing challenge to wealthy countries, and not enough is being done.

"It's worse than the slavery of those whose slaves were taken from Africa and brought to other countries," Martino said. "In a world which proclaims human rights left and right, let's see what it does about the rights of so many human beings which are not respected, but trampled."

Cardinal Martino was releasing an annual papal message on the problems faced by migrant communities. He singled out the buying and selling of women for prostitution and minors for child labour as the most pressing issues that need to be addressed.

The papal message says refugee women are very vulnerable to being sold as sex workers, particularly those in refugee camps.

"There is the risk of women and children [in the camps] being involved in sexual exploitation merely as a survival mechanism," the message says.

A recent report from the U.S. State Department estimated that as many as 800,000 people are trafficked across international borders each year, 80 per cent of them women, and more than half under 16. 

Scholars estimate that 12 million Africans were sold into slavery over many centuries. Most went to the Caribbean, Brazil and the United States.

With files from the Associated Press