Turkey has to provide serious evidence that Turkish cleric Fethullah Gulen was behind a coup attempt last weekend, Secretary of State John Kerry said.

Kerry said that during phone calls with his Turkish counterpart, he made clear that mere allegations are not enough for extradition, Associated Press reported.

“We have a very strict set of requirements that have to be met for an extradition to take place,” he said.

Kerry said he had not yet seen the documents and other U.S. officials have not yet said whether they constitute a formal extradition request.

Secretary of State said Washington is following developments in Turkey and reiterated support for the elected authorities.