BRUSSELS (AP) — The Ukrainian armed forces have made significant early gains in their counter-offensive against Russian forces in southern and eastern Ukraine but fighting appears set to drag on for months, U.S. Secretary of State Antony Blinken and the head of NATO said Friday.

Blinken, who was at NATO headquarters to brief the 29 U.S. allies after a trip to Kyiv on Thursday, said that the six-month war in Ukraine is entering a critical period and he urged the conflict-torn country’s Western backers to maintain their support in coming months and through the winter.

“The initial signs are positive, and we see Ukraine making real, demonstrable progress in a deliberate way,” Blinken said, referring to the recent push into southern Ukraine and the eastern Donbas region by local forces.

“But this is likely to go on for some significant period of time,” he said. “There are a huge number of Russian forces that are in Ukraine, and unfortunately, tragically, horrifically, President (Vladimir) Putin has demonstrated that he will throw a lot of people into this at huge cost to Russia.”

NATO Secretary-General Jens Stoltenberg said the invasion launched by Putin in February is “entering a critical phase.”

“Ukrainian forces have been able to stall Moscow’s offensive in Donbas, strike back behind Russian lines and retake territory,” he said.

But Stoltenberg warned that allied unity will be tested in coming months, “with pressure on energy supplies and the soaring cost of living caused by Russia’s war.” He renewed calls for allies to supply special uniforms, generators, tents and equipment to help Ukraine’s army weather the coming winter.