Over 547 irregular migrants were held in Istanbul and referred to the repatriation center, Istanbul Governor Ali Yerlikaya said Wednesday.

The migrants, including 167 Afghan nationals, were held on Tuesday during search operations across Istanbul's 18 districts, Yerlikaya tweeted.

Turkey has been a key transit point for asylum-seekers attempting to cross into Europe to start new lives, especially those fleeing war and persecution.

Turkey, which already hosts 4 million refugees – more than any country in the world – is taking new security measures within and on its borders to prevent a fresh influx of migrants.

Concerns have risen over a possible spike in migrants from Afghanistan due to the United States' pullout from the country after two decades.

Within the scope of fighting irregular migration, a 243-kilometer (151-mile) concrete wall, topped with barbed wire and surrounded by trenches, is being erected along Turkey’s 534-kilometer frontier with Iran.