Turkish police on Tuesday detained at least 20 people in a nationwide operation aimed at cracking down on the trafficking of migrants seeking to reach the European Union, reports said.

The suspects were detained in morning raids in five regions, from Izmir on the Aegean to Sanliurfa close to the Syrian border and Samsun on the Black Sea following a three month investigation, the Dogan news agency said.

It said that the suspected traffickers, who are citizens of Turkey, Syria and Iraq, were believed to have smuggled  migrants from Turkey's coast across the Aegean Sea in exchange for money.

With the operation ongoing, the number of those detained could rise, it added.

Materials seized in the raids included lists of migrants, money counting machines and hard currency, Turkish media reports said.

Turkey has over the last year emerged as the major hub for refugees and migrants from Syria, Iraq, Afghanistan, Eritrea and other troubled states trying to reach the European Union via the Greek islands.

Around one million migrants crossed the Aegean to Greece in 2015, prompting rattled EU leaders to seek Turkey's help for a solution.

Under a landmark deal thrashed out between the EU and Ankara earlier this month, all migrants arriving on the Greek islands are now designated for return to Turkey.

The numbers of people reaching Greece from Turkey have declined sharply since the EU-Turkey deal went into effect on March 20.

But several EU leaders had also underlined it was essential for Turkey to crack down on the people smuggling business, which analysts say had been allowed to operate with relative impunity in Turkey.

Smugglers would ask for up to several thousand dollars for a place in a potentially unseaworthy boat for the risky crossing to the Greek islands.