Migrants have made some 800,000 "illegal entries" to the European Union so far this year, the head of the bloc's border agency Frontex says.
Warning the influx of migrants has probably not yet "reached its peak", Fabrice Leggeri called for European states to detain unsuccessful asylum seekers so they can be "rapidly" sent back to their countries of origin.
"EU states must prepare for the fact that we still have a very difficult situation ahead of us in the coming months," Leggeri said in an interview with German newspaper Bild.
Last month, Frontex said that 710,000 migrants had entered the EU in the first nine months of the year but cautioned that many people had been counted twice.
The agency said on October 13 that "irregular border crossings may be attempted by the same person several times".
"This means that a large number of the people who were counted when they arrived in Greece were again counted when entering the EU for the second time through Hungary or Croatia," explained the agency.
According to the most recent figures from the UN refugee agency, more than 744,000 people have made the perilous journey across the Mediterranean this year, the majority to Greece.