President Volodymyr Zelensky vowed Sunday that his forces "will not give away the south to anyone" after his first visit to the southern frontline, as NATO's chief warned the war in Ukraine could last "for years," according to AFP.

Making a rare trip outside Kyiv, where he is based for security reasons, Zelensky traveled to the hold-out Black Sea city of Mykolaiv and visited troops nearby and in the neighboring Odessa region for the first time since the Russian military operation.

"We will not give away the south to anyone, we will return everything that's ours and the sea will be Ukrainian and safe," he said in a video posted on Telegram as he made his way back to Kyiv.

He said he talked with troops and police during his visit.

"Their mood is confident, and looking into their eyes it is obvious that they all do not doubt our victory," he said.

While Zelensky remained defiant, NATO Secretary-General Jens Stoltenberg warned that "we must be prepared for this to last for years."

Speaking to German daily newspaper Bild, Stoltenberg said "We must not weaken in our support of Ukraine, even if the costs are high -- not only in terms of military support but also because of rising energy and food prices."

Russian forces have directed their firepower at the east and south of Ukraine in recent weeks since failing in their bid to take the capital Kyiv after the lightning February 24 military operation.

"The losses are significant. Many houses were destroyed, civilian logistics were disrupted, there are many social issues," Zelensky said.

"I have commissioned to make assistance to people who have lost loved ones more systemic. We will definitely restore everything that was destroyed. Russia does not have as many missiles as our people have the desire to live."

Mykolaiv is a key target for Russia as it lies on the way to the strategic Black Sea port of Odessa.

Zelensky surveyed the city's badly damaged regional administration building and met officials in what appeared to be a basement where he handed out awards to soldiers, in a video released by his office.