UN chief Antonio Guterres was expected in war-scarred southern Ukraine's main port of Odessa on Friday, a day after he said Turkey and Ukraine hoped to scale up their landmark grain export deal with Russia.

The visit was due as Russian forces pursued a relentless bombardment campaign in Ukraine's eastern Donbas region, with Kyiv reporting five people killed across the industrial province of Donetsk in the last 24 hours.

As part of the UN chief's visit to Ukraine, he met with Turkish leader Recep Tayyip Erdogan, who helped broker the grain deal inked in Istanbul, and Ukrainian leader Volodymyr Zelensky.

At a joint press conference with the three leaders, Guterres said the United Nations aimed to "scale-up" operations under the deal ahead of winter.

The only significant agreement between Russia and Ukraine since Moscow invaded in February, has so far seen 25 boats carrying some 600,000 tonnes of agricultural products depart from three designated ports, Kyiv has said.

Zelensky told Erdogan and Guterres at the press conference he believes it is a "global need to increase the number of vessels that safely export Ukrainian food".

"Our state is ready to be and will be the guarantor of global food security," Zelensky added.

Guterres is expected to head to Turkey after Odessa to visit the Joint Coordination Centre, the body tasked with overseeing the accord.

The agreement between Kyiv and Moscow to clear exit corridors from three Ukrainian ports, including Odessa, has brought some relief to concerns of global food shortage with the warring countries among the world's leading producers.

It looked to be on the verge of collapse when, one day after it was signed, Russian missiles hit the port of Odessa, spurring an outcry from global leaders.

It has since held but the accord has brought little respite along the sprawling front lines in eastern Ukraine, where Russian forces have edged slowly forward after nearly six months of fighting.

The primary tool of Moscow's forces has been artillery barrages, and recent bombardments over the Donetsk region -- which has been partially controlled by Russian proxies since 2014 -- left several dead.

The Ukrainian head of the region, Pavlo Kyrylenko, said on social media Friday that Russian strikes had killed five people and injured 10 more in three settlements.

Strikes early Friday in Kharkiv, Ukraine's second-largest city, left one person dead and damaged a school and a private business, the head of the region said. Russian strikes around Kharkiv have killed more than a dozen people over the last two days.

But concerns in recent days have centred around the Russian-controlled Zaporizhzhia nuclear power station in south Ukraine, where both sides have traded blame for strikes and accused the other of planning attacks.

The flare-up around Europe's largest nuclear facility has sparked urgent warnings from world leaders, and Guterres cautioned during talks with Erdogan that any damage to the plant would be akin to "suicide".

The Turkish leader meanwhile said: "We are worried. We do not want another Chernobyl" referring to the 1986 nuclear disaster.

Moscow dismissed Ukrainian allegations Thursday that its forces had deployed heavy weapons at Zaporizhzhia and instead accusing Kyiv of preparing a "provocation" there that would see Russia "accused of creating a man-made disaster at the plant".

Kyiv, however, insisted it was Moscow that was planning a "provocation" at the facility.