STORY: His remarks came after the Kremlin warned that Western arms supplies to Ukraine, including heavy weapons, posed a threat to the security of the European continent.
"We need to be prepared for the long term. It's a very unpredictable and fragile situation in Ukraine. But there is absolutely the possibility that this war will drag on and last for months and years," Stoltenberg told a youth summit in Brussels.
He said the West would continue to put maximum pressure on Russia's President Vladimir Putin to end the war that Moscow calls a "special military operation" - by imposing sanctions and by offering economic but also military aid to Ukraine.
"NATO allies are preparing to provide support over a long period of time and also help Ukraine to transit or move from old Soviet-era equipment to more modern NATO standard-weapons and systems," Stoltenberg said.