A powerful, stable, independent Ukraine is the key to Euro-Atlantic security, NATO Secretary General Jens Stoltenberg has said.

"Winter is coming and it will be hard. And what we see now is a grinding war of attrition. This is a battle of wills and a battle of logistics. Therefore, we must sustain our support for Ukraine for the long-term, so that Ukraine prevails, as a sovereign independent nation. A strong, stable and independent Ukraine is essential to the Euro-Atlantic security. NATO, together with Ukraine, has supported its 30 years of independence, we stand with Ukraine now, we will continue to stand with Ukraine for as long as it takes," Stoltenberg said at the Second Summit of Crimea Platform on Tuesday.

He said NATO helps Ukraine to defend itself, this is its sovereign right.

"We are committed to our partnership with Ukraine for the long term. We will help Ukraine transition from Soviet-era to NATO-standard equipment, continue your defense and security sector reform, and help you on the path of post-war reconstruction," Stoltenberg said.

The Secretary General said he is constantly contacting the leaders of countries so that they provide more military equipment and more ammunition as soon as possible.

"NATO stands in full solidarity with Ukraine. Allies fully support Ukraine's sovereignty and territorial integrity, your right to self-defense and to choose your own path. President Putin started this unjustified war, he must stop this war and withdraw his forces from Ukraine," the Secretary General said.