"They have shown a great willingness to tolerate losses and suffering," Jens Stoltenberg said, as quoted by the Reuters news agency.
Speaking at a business conference in Norway, Stoltenberg warned that there was "no indication that President Putin has changed his plans and goals" in Ukraine.
"So it's dangerous to underestimate Russia," he told the conference, according to Reuters.
Stoltenberg said on New Year's Day that Western countries must be prepared to provide long-term support to Ukraine as Russia's war shows no signs of ending.
He told British broadcaster BBC in an interview that Russia is "prepared to continue the war and also try to potentially launch a new offensive."
In another recent interview, Stoltenberg urged allies to supply more weapons to Ukraine to help it defend itself against Russian attacks.
"I call on allies to do more," Stoltenberg said in an interview published last week by German news agency DPA.
He added that it was especially important that Ukraine receive new supplies of ammunition and spare parts for military equipment.
Stoltenberg said last week that "military support for Ukraine is the fastest way to peace."
Thursday is day 316 of Russia’s war in Ukraine.
Source: PAP, Reuters