NATO Secretary-General Jens Stoltenberg has categorically ruled out any role for the military organization in setting up and policing a no-fly zone over Ukraine to protect against Russian airstrikes.

Stoltenberg says “NATO should not deploy forces on the ground or in the air space over Ukraine because we have a responsibility to ensure that this conflict, this war, doesn’t escalate beyond Ukraine.”

Ukrainian President Volodymyr Zelenskyy has repeatedly appealed for NATO to set up a no-fly zone given Russia’s air superiority, as civilian casualties mount three weeks into the war.

Speaking Wednesday after chairing a meeting of NATO defense ministers, Stoltenberg conceded that “we see human suffering in Ukraine, but this can become even worse if NATO (takes) actions that actually turned this into a full-fledged war between NATO and Russia.”

He says the decision not to send air or ground forces into Ukraine is “the united position from NATO allies.” Earlier Wednesday, Estonia urged its 29 NATO partners to consider setting up a no-fly zone.