"The counteroffensive, it is difficult. People should never think that this is an easy walkover," Admiral Rob Bauer, the chair of NATO's Military Committee, told journalists.
"There's a considerable number of Russians in Ukraine. There are considerable defensive obstacles."
Kyiv has admitted that its push in the south of the country is tough in the face of deeply entrenched Russian positions, but says its forces are recapturing territory.
Bauer insisted "it is not a surprise that it is not going fast".
"We should not comment on Ukraine like they should go faster or it's disappointing they're not going fast. It is extremely difficult, this type of operation," he said.
He said Ukrainian commanders were right to be "cautious" probing for weaknesses in the Russian lines, as they risk losing a lot of forces in full-on assaults.
The war in Ukraine has propelled NATO to carry out the biggest overhaul of its eastern defences since the end of the Cold War over three decades ago.
Leaders from the Western military alliance meeting in Lithuania next week are expected to sign off on new military regional plans detailing how NATO would respond in case of a Russian attack.
Bauer said NATO believed Russia's military would "bounce back" from the mauling its forces have taken in Ukraine and reconstitute in the years to come.
"We will continue to look at them as a serious threat," he said.
"They might not be 11 feet tall, but they're certainly not two feet tall."
He said the time it takes Moscow to rebuild could give NATO breathing space as it will take "a considerable number of years" to reach the larger number of higher readiness forces needed to make its new plans fully operational.
In the meantime, the Dutch commander said NATO countries needed to do all they could to keep backing Kyiv.
"If Ukraine loses that war, and that is not the end of instability. That is the start of more instability," he said.
He said bulking up NATO's posture "costs money" and has previously warned that delays to boosting military spending could slow that down.
NATO leaders are expected to agree to increase their defence spending targets to a minimum of two percent of gross domestic product at the upcoming summit.
However some allies below that figure do not want the commitment to get to the two-percent target to be immediate.
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