Stoltenberg, who was meeting with Biden in the White House later Tuesday, told CNN that it was "still early days" for the offensive, which he characterized as "difficult."

Kyiv's Western-backed army is attempting to assault long-prepared Russian defensive lines stretched across the east and south of war-torn Ukraine.

President Vladimir Putin ordered an invasion in February 2022, quickly seizing large areas of territory, but meeting fierce -- and growing -- resistance. It remains unclear how much the Ukrainian counteroffensive can achieve in terms of recovering lost lands, which include the entire Crimean peninsula.

"They have the right... to liberate their own land," Stoltenberg said.

He said that "the more land they gain, the more likely it is that President Putin will understand that he has to sit down at the negotiating table and agree to a just and enduring peace in Ukraine."

Referring to the politics around who will replace him after his term as NATO chief ends in October, Stoltenberg said: "I'm absolutely confident that they will find an excellent successor. My focus now is to lead this alliance until my tenure ends, because we are in the middle of a war in Europe."

The meeting with Biden was scheduled for Monday but postponed after the US president had to undergo dental work. Preparations are underway for a NATO summit in July in the Lithuanian capital Vilnius.