Stoltenberg on Monday said at a press conference that he had contacted the Dutch and Turkish governments after Turkish President Recep Tayyip Erdogan had said the Netherlands is acting like "Nazis and Fascists", Efe news reported.
"Robust debate is at the heart of our democracies but so is mutual respect," Stoltenberg told reporters. "I would encourage all allies to show mutual respect, to be calm and have a measured approach and to contribute to de-escalate the tensions."
"It's important that we now focus on everything that unites us," the Secretary General said.
He said Turkey's presence in NATO was "good for Turkey but also for Europe and for the rest of the alliance" in face of the instability in neighbouring countries such as Irak or Syria.
A diplomatic spat between the Netherlands and Turkey gained momentum over the weekend after Dutch authorities denied landing permission for a plane carrying Turkey's foreign minister who was planning to attend a referendum campaign in Rotterdam.
The Turkish government recently launched a verbal attack on the German government after similar events were cancelled in Germany.
"They are Nazi remnants, they are fascists," Erdogan said and went on to describe the Netherlands as a "banana republic" and warned of sanctions in retaliation.
Turkey is preparing for a referendum that could pave the way for Erdogan to assume more powers as president.