The European Union's law enforcement agency has warned that refugees stranded across Europe are prey to being forced into prostitution and slave labor by organized criminal gangs.

Europol’s Chief of Staff Brian Donald said on Sunday that criminal gangs identify unaccompanied children and young women to exploit them for prostitution and illegal labor.

Addressing an international conference on human trafficking in Madrid, Donald said children travelling alone or in groups without adults are particularly vulnerable.

“Modern, enterprising, organized criminal gangs go where the opportunity is high and the risk is low,” he said, adding, “In the coming years, the phenomenon of migrants being exploited once they’ve reached the European Union will become more visible.”

Donald said that European law enforcement agencies have recorded at least 7,000 unaccompanied minors among refugees and migrants entering Europe in the recent past.

“We need to keep on top of the nexus between smuggling gangs and trafficking gangs,” he said.

Refugee arrivals hits new high

Meanwhile, the United Nations refugee agency (UNHCR) says more than 218,000 asylum seekers arrived in Europe by sea in October, setting a new monthly record.

Adrian Edwards, a spokesman for UNHCR's office, said on Monday that the figure is the highest for any month to date and roughly the same as the entire total for 2014.

The UNHCR puts September arrivals at 172,843 while about 219,000 reached European shores by sea in 2014.

According to the UNHCR, more than 700,000 asylum seekers have arrived in the continent since the beginning of 2015. Over half of the new arrivals are from Syria and 18 percent from Afghanistan, while children account for 20 percent of the total figure.

The International Organization for Migration announced last week that the flow of refugees shows no signs of abating despite the worsening weather conditions. This is considered the biggest refugee crisis in Europe since the Second World War.