A joint statement from the Visegrad Four (V4) presented at the informal EU summit in Bratislava on Friday says the EU's migration policies should be based on the principle of flexible solidarity It is necessary to halt the influx of illegal migrants, the V4 countries, namely the Czech Republic, Hungary, Poland, and Slovakia, stressed, adding that a boost in the cooperation with third countries, including Turkey, and safeguarding of the outer Union border must be among the key instruments.