The number of convictions for trafficking in Europe has fallen by a quarter, despite an increase in the number of victims and a global drive to tackle the abuse.

A report by the UN found that 742 people in Europe were convicted for trafficking offences in 2016, the latest year for which there is available data, compared with 988 in 2011. The number of identified victims increased from 4,248 to 4,429 over the same period.

The fall in convictions could reflect the hardening of border controls across Europe, and a failure to recognise victims of abuse, said Kevin Bales, professor of contemporary slavery at the University of Nottingham.

“People who might have been detected as victims of a crime in the past in a lot of western European countries, are now being treated as not victims of a crime but as illegal migrants and are being deported or dumped that way,” he said.

The drop in convictions is also likely to be due to continued difficulties authorities face when building cases under modern slavery laws, which are still relatively new.

“The convictions either are hard to make, and we know that to be true, under some of the newer slavery and trafficking laws, or they are choosing to prosecute under a statute with which they are more likely to get a successful prosecution,” Bales said.

Authorities may instead pursue a conviction for crimes such as grievous bodily harm, where evidence may be easier to gather, he added.

Globally, the number of identified victims of trafficking increased by 40% in the five years to 2016, to 24,000. However, this still represents only a fraction of the more than 40 million people worldwide who are believed to be affected by slavery.

Though convictions have increased globally, there are still “vast areas of impunity”, according to the report by the United Nations Office on Drugs and Crime (UNODC).

The study warned that more countries were experiencing some form of violent conflict than at any other time in the previous 30 years, placing even greater numbers at risk.

“Armed groups use [trafficking] for their strategy,” said Kristiina Kangaspunta, chief of the Crime Research Section at UNODC. “They are exploiting, abusing people to show they have control over the community, or to increase their force, either recruiting child soldiers or giving sex slaves as a reward for their recruitment.”

Sexual exploitation accounts for the majority, 59%, of identified trafficking cases. Forced labour accounts for around a third of all detected cases.

Globally, around half of all trafficked people are adult women, while a fifth are girls. Across sub-Saharan Africa, children accounted for the majority, 55%, of victims in 2016, with girls and boys equally targeted.

For the first time, the report found that the majority of victims worldwide – almost six in ten – are people trafficked within their own country. This may reflect increased border controls, as well as greater awareness of the different forms of trafficking, and the introduction of new, more comprehensive legislation in many countries.

“It is very important to recognise that we always talk about trafficking as if it is one type of crime, but it is not,” said Kangaspunta. “It can be many types of phenomena: it can be cases of one person trafficked by another person, [such as] a family member, or it can be big organised crime, and anything in between.”

The number of detected victims for organ trafficking was “very limited”, the report added. About 100 such victims, all adults, were detected and reported to UNODC between the years 2014-2017.