Nato's chief warned that the war in Ukraine could last "for years" as President Volodymyr Zelensky vowed on Sunday that his forces would not give up the south of the country to Russia after he visited the frontline there.

Ukraine said it had also repulsed fresh attacks by Russian forces on the eastern front, rocked by weeks of fierce battles as Moscow tries to seize the industrial Donbas region.

While Ukraine remained defiant, Nato Secretary-General Jens Stoltenberg urged that Western countries must be ready to offer long-term military, political and economic support to Kyiv through a grinding war.

"We must be prepared for this to last for years," Stoltenberg told German daily newspaper Bild. "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."

British Prime Minister Boris Johnson issued a similar warning, urging sustained support for Kyiv or risking "the greatest victory for aggression" since World War II.

"Time is now the vital factor," Johnson wrote in an article for Britain's Sunday Times after making his second visit to Kyiv, calling for the West to ensure Ukraine has the "strategic endurance to survive and eventually prevail".

Ukraine has repeatedly urged Western countries to step up their deliveries of arms since the February 24 invasion, despite Russian warnings that it could trigger wider conflict.

Zelensky made a rare trip outside Kyiv on Saturday to the hold-out Black Sea city of Mykolaiv, and visited troops nearby and in the neighbouring Odessa region for the first time since the Russian campaign began.

"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.

But Zelensky admitted that losses were "significant", adding: "Many houses were destroyed, civilian logistics were disrupted, there are many social issues."

Russia's defence ministry said on Sunday it had launched missile strikes during the past 24 hours, with one attack by Kalibr missiles on a top-level Ukrainian military meeting near the city of Dnipro killing "more than 50 generals and officers".

It said it also targeted a building housing western-delivered weapons in Mykolaiv, destroying "ten 155 mm howitzers and around 20 armoured vehicles supplied by the West to the Kyiv regime over the last ten days". (AFP)