Turkish police detained Tuesday over 200 people in a major nationwide crackdown on illicit money transfers to individuals of Iranian-origin living in the United States.

An Istanbul court issued arrest warrants for a total of 417 suspects and have so far rounded up 216 in a nationwide operation carried out in 40 cities, the official Anadolu news agency reported.

The suspects are facing charges including criminal conspiracy and violating measures to prevent terrorism financing, Anadolu said.

There were no further details on who the suspects are.

An investigation found that from the beginning of 2017 some 2.4 billion lira ($400 million) had been transferred from Turkey to a total of 28,088 accounts abroad from several banks, it added.

Those conducting the money transfers earned “commissions”, Anadolu said, citing the investigation, while the large majority of the recipients of the funds were people of Iranian origin living in the United States.

There were no further details on who these individuals were and if the transfers had an overall aim.

The arrests come as the United States is in the process of reimposing sanctions on Iran after Washington pulled out of a 2015 international deal limiting the nation's nuclear programme, including on Iran's ability to use the international financial system and export oil.

They also come several months after Turkey-US relations hit a rough patch over the detention of a US pastor, with US President Donald Trump making comments that sent the Turkish lira tumbling.

Sanctions busting has also been a source of tension between Washington and Ankara.

US authorities earlier this year convicted Mehmet Hakan Atilla, deputy director general of Turkish lender Halkbank on charges of helping Iran evade US sanctions on billions of dollars of oil proceeds.

In the case, Iranian-Turkish trader Reza Zarrab, who was once close to the Ankara authorities, turned whistleblower and gave evidence accusing the Turkish leadership of being complicit in the scheme. Turkey depends on foreign suppliers including Iran for its energy needs.

The country imports oil and natural gas from Russia as well as from neighbouring Iran.