International organised crime is worth 870 billion dollars, the head of the UN's office on drugs and crime (UNODC) said Monday, urging greater coordination in fighting it.

"We are able to quantify the cost of transnational organised crime, it is $870 billion (672 billion euros)," UNODC executive director Yury Fedotov told a crime conference in Vienna.

"But we cannot calculate the misery and suffering caused to millions of people by these illicit activities," he added.

The perpetrators are "smart, sophisticated and largely opportunistic," he said, calling for better international cooperation to tackle highly mobile organised crime groups.

Some 800 ministers and civil society representatives are attending the week-long conference on transnational crime in the Austrian capital.