Jacques Barrot told a Brussels news briefing after a visit to Athens that he had promised the Greek government financial help to deal with the problem and more active EU talks with Turkey to ensure better supervision of illegal migration.
"Because we can indeed imagine a major risk of destabilising Greek democracy through migrations that are absolutely uncontrolled and uncontrollable," he said.
Barrot said the EU's executive European Commission would work more actively towards an agreement on readmission to Turkey of illegal migrants and on better monitoring of illegal departures from the Turkish coast.
"Turkey has to help us to fight against facilitators and traffickers," he said.
"We cannot do nothing and we need to obtain, with Turkey, much firmer and stricter negotiations ... we will, for our part, help Turkey through readmission agreements we hope to sign with Pakistan and maybe other Asian countries," he added.
In Athens on Tuesday, Barrot had accused Turkey of turning a blind eye to trafficking of illegal migrants to Greece.
Greece said this month that it had arrested about 47,000 illegal immigrants coming from Turkey, an EU candidate country, last year. Greece says Ankara must take back illegal migrants who have crossed Turkey.
Ankara says the migrants come from countries such as Iraq and Pakistan and it should not have to handle those crossing Turkey to reach the wealthy EU.
The conservative Greek government, stung by far-right gains in an EU Parliament election, said this month it would get tougher on illegal migration including by detaining illegal migrants for up to 12 months, instead of three currently.
(EurActiv with Reuters.)