He noted this at a joint press conference with Ukrainian President Petro Poroshenko in Kyiv on Monday.
"Russia must withdraw thousands of its soldiers from Ukraine and stop supporting the militants, with command and control, and military equipment," Stoltenberg said.
He said that the situation in east Ukraine remained tense and that the ceasefire was not holding.
Stoltenberg also expressed concern about increasing threats against OSCE monitors.
"NATO stands firm in our support for Ukraine’s sovereignty and territorial integrity," he said.
As reported, Chief of the General Staff and Commander-in-Chief of the Ukrainian Armed Forces Viktor Muzenko said on July 7 that the number of Russian troops staying in the occupied territories of Ukraine currently exceeded 30,000 people.