German and Turkish police arrested 15 people suspected of people smuggling in a joint operation to break up a ring believed to have trafficked more than 1,700 Syrian refugees to Europe.

The suspects were almost all Syrians, and at least two of the five people arrested in Germany were Syrian asylum seekers, senior police officers told reporters in Potsdam, near Berlin, on Wednesday.

"This joint operation to dismantle an international traffickers' ring is the biggest one to date," the president of Germany's Federal Police, Dieter Romann, said.

German Chancellor Angela Merkel is under pressure to slow a surge of migrants after more than 1 million arrived in Germany last year, fleeing war and poverty in Syria and other countries in what is Europe's biggest refugee crisis since World War II.

Traficking operations are thought to account for most of the migrants arriving in Europe across the Mediterranean or Aegean seas, the International Organization for Migration said in December.

German police said they had made five arrests on Tuesday after raids in six federal states, while in Turkey simultaneous operations in different regions led to 10 arrests.

The arrests were the result of an investigation launched in December 2014 after three ships, which had been bound for the scrap heap, set sail from the Turkish coast on auto-pilot.

Packed with Syrian refugees, the ships ended up stranded in the Mediterranean, before being rescued by Italian coast guard elements.

The smugglers charged the refugees wanting to make the dangerous sea journey to Europe between $4,500 to $6,000, netting the traffickers about $9.3 million from 1,766 asylum seekers, according to police calculations.

The suspects arrested in Germany face prison terms of up to 10 years if found guilty.