Turkish prosecutors issued detention warrants for 254 Istanbul municipality and ministry staff over alleged links to the US-based cleric accused of orchestrating last year’s attempted coup, state media said on Tuesday.

The magnitude of the latest operations - which targeted current as well as previously dismissed staff - mark a fresh escalation against the movement of Muslim preacher Fethullah Gulen, more than 14 months after the failed putsch.

The Interior Ministry said on Monday nearly 1,000 people were detained over the previous week on allegations of ties to what Ankara calls the “Gulenist Terror Group”.

In Istanbul, prosecutors ordered the arrest of 112 current and former staff of municipalities in Turkey’s largest city, state-run Anadolu agency reported. They were accused of being users of Bylock, an encrypted messaging app which the government says was used by Gulen’s followers.

Thirty-five of these suspects have so far been detained in the operation, stretching across eight provinces, Anadolu said.

Prosecutors in Ankara ordered the detention of 142 staff of the education and sport ministries, the agency said. The vast majority of them had already been dismissed from their jobs over alleged Gulen ties.

Gulen, who has lived in self-imposed exile in the United States since 1999, has denied involvement in the 15 July coup, in which 250 people were killed.

Since then more than 50,000 people have been jailed pending trial over links to Gulen, while 150,000 people have been sacked or suspended from jobs in the public and private sectors.

Rights groups and some of Turkey’s Western allies have voiced concern about the crackdown, fearing the government is using the coup as a pretext to quash dissent.

The government says only such a purge could neutralise the threat represented by Gulen’s network, which it says deeply infiltrated institutions such as the army, schools and courts.