In a telephone call with President Erdogan, Jens Stoltenberg expressed his full support for the Turkish government.
However, he also warned that "there can be no room for military coups in a Nato state."
It comes amid fevered speculation that the embattled country risks losing its membership of Nato, as well as escalating tensions between Turkey and the United States, where the Muslim cleric accused of planning the coup, Fethullah Gülen, currently resides.
Jens Stoltenberg earlier warned President Erdogan that Turkey must maintain "full respect" for democracy.
"Being part of a unique community of values, it is essential for Turkey, like all other allies, to ensure full respect for democracy and its institutions, the constitutional order, the rule of law and fundamental freedoms," he said.
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