BERLIN (Reuters) - German police on Wednesday raided the apartments of four imams suspected of conducting espionage on behalf of the Turkish government against followers of U.S.-based cleric Fetullah Gulen, accused by Ankara of organising a failed coup last July.

The new Central Mosque is photographed in Cologne, Germany, February 15, 2017. REUTERS/Thilo Schmuelgen

The raids, in the states of North Rhine-Westphalia and Rhineland-Palatinate, could further strain relations between the two NATO allies, which need each other to resolve issues ranging from militant Islamist attacks to Europe’s migrant crisis.

The Federal Public Prosecutor’s Office (GBA) said in a statement that the imams had acted on an order issued on Sept. 20 last year by the Turkey-based Diyanet religious authority, which said the Gulen movement was behind the putsch attempt.

German Justice Minister Heiko Maas said the four imams were members of Ditib, Germany’s largest association of mosques, which brings imams from Turkey to serve the community of some three million people with a Turkish background who live here.

“It is very clear that the influence of the Turkish state on Ditib is big. The association must plausibly disengage itself from Ankara,” Maas said in a statement.

A Ditib spokesman rejected the allegations and underlined that the investigations were not being carried out against the association or its mosques.

“The investigations are looking into individual imams who are accused of collecting and passing on information illegally,” spokesman Bekir Alboga said, adding that Ditib was not the official employer of the imams.

“We represent almost 50 percent of the mosques and a majority of the Muslims in Germany who pose no kind of danger,” he added.

Last month, the GBA opened an investigation into Turkish intelligence operations on German soil after a lawmaker filed a criminal complaint. Austria is also investigating whether Turkey has been operating an informer network targeting Gulen followers on its soil, via its embassy in Vienna.

Turkey has accused Germany of harbouring militants from the Kurdistan Workers Party (PKK) and far-leftists of the DHKP-C, which has carried out attacks in Turkey. German officials reject the accusation.

The GBA said Wednesday’s searches were aimed at finding more evidence to link the suspects to espionage activities.

“The suspects are suspected of having collected information about members of the so-called Gulen movement and passed it on to the general consulate in Cologne,” the GBA said.

Turkish President Tayyip Erdogan accuses Gulen of masterminding the abortive coup, in which at least 240 people were killed, and wants the United States to extradite him. Gulen denies any involvement in the coup attempt.

Since July Turkey has arrested tens of thousands of people over suspected ties to Gulen in a wide-ranging crackdown criticised by Germany and other European Union nations.