"It's up to Ukrainians, and Ukrainians alone, to decide when the conditions for negotiations are in place, and to decide at the negotiating table -- in the event of eventual negotiations -- what an acceptable solution is," Stoltenberg said.

"Our job is to support them," he added.

Stoltenberg was speaking at an event in Arendal, a small town in his native Norway, where Norwegian decision-makers are gathered this week.

Two days earlier at the same event, Stian Jenssen, the director of the private office of the NATO secretary general, suggested that one solution to the conflict could be that Ukraine be granted NATO membership in exchange for ceding some of its territory to Russia.

Jenssen later walked back his remarks.

"NATO's policy is unchanged," Stoltenberg said.

"We support Ukraine in its internationally recognised borders... (its) sovereignty, and (its) territorial integrity".

During a NATO summit in Vilnius in early July, Ukraine was offered long-term military support but no NATO membership in the short term.