[A migrant boy walks in the transit center for refugees in Sid, about 100 km west from Belgrade, Serbia, Thursday, March 3, 2016. Greece's prime minister Alexis Tsipras has called for sanctions to be imposed on European Union states that refuse to take in their share of the hundreds of thousands of refugees flowing into the continent through his country. (AP Photo/Darko Vojinovic)]ATHENS, Greece (AP) — The European Union's leader chose the continent's main gateway for immigrants Thursday to bluntly warn economic migrants not to even think of coming to Europe, while promising to work for a solution to the more immediate problem of refugees.