More than two million children have fled Ukraine since Russia invaded on February 24, according to the 42-page report compiled by the Dutch-based La Strada anti-trafficking organisation and commissioned by Britain's Freedom Fund.

"People fleeing the war, mainly women and children, are extremely vulnerable and face increased risk of sexual and labour exploitation as organised criminal groups and individual profiteers take advantage of the turmoil," the groups said in a statement.

There are already signs of human trafficking activity since the war started "and it is likely that these risks will increase in the coming period, as the war continues," the report said.

The study, based on research done in March and April using UN and other figures, called for "urgent action" by European governments and international agencies "to invest in the promotion of the registration of refugees to ensure swift access to protection measures".

In particular, unaccompanied minors and children separated from their families should be identified and registered "as early as possible" and tracked through transit points.

It urged countries to work together to trace missing children and introduce mandatory checks for unaccompanied minors.

It also called for the prevention of adoptions, which could lead to child sexual exploitation or illegal adoptions, recommending, "Efforts should first focus on reunification of children with their families."

Other vulnerable groups including women, non-Ukrainian citizens and members of the Roma community, should be given proper information about the dangers of trafficking, it said.