KYIV (Reuters) - Ukraine's top commander warned on Friday of "heavy battles" looming on the war's new front in the northeastern Kharkiv region as Russian President Vladimir Putin said Moscow was carving out a "buffer zone" in the area.

Russian forces attacked the Kharkiv region's north last Friday, making inroads of up to 10 kilometres (6 miles) and unbalancing Kyiv's outnumbered troops who are trying to hold the line over a sprawling front nearly 27 months since the full-scale invasion.

Colonel-General Oleksandr Syrskyi said the attack had expanded the area of hostilities by around 70km and that Russia had launched its incursion ahead of schedule when "it noticed the deployment of our forces".

"We understand there will be heavy battles and that the enemy is preparing for that," the head of the Ukrainian armed forces wrote in a statement on the Telegram app.

Speaking during a state visit to China, Putin said Moscow's forces were creating a "buffer zone"to protect Russian border regions, but that capturing the city of Kharkiv, Ukraine's second largest, was not part of the current plan.

The Russian leader told a news conference the assault was a response to Kyiv's shelling of Russian border regions such as Belgorod.

"Civilians are dying there. It's obvious. They are shooting directly at the city centre, at residential areas. And I said publicly that if this continues, we will be forced to create a security zone, a buffer zone. That is what we are doing," Putin said.

Russian forces were able to advance 10 kilometres in one place, but Ukrainian forces have "stabilised" the front, President Volodymyr Zelenskiy told Ukrainian media outlets in comments published on Friday.


Moscow's forces are mounting their heaviest assaults in the eastern Donetsk region, according to data compiled by the Ukrainian General Staff, which said the eastern Pokrovsk front had faced the most regular assaults in recent days.

In his comments, Syrskyi said Ukrainian forces were preparing their defensive lines for a possible new Russian assault on the Sumy region, which would mark another front more than a hundred kilometres to the north of Kharkiv.

Kyiv has warned that Russia has small units of forces near the Sumy region.

Volodymyr Artiukh, head of the Sumy region's military administration, said Russian military activity along the northern Ukrainian region was at a high level.

"We note that the actions (of Russian forces) are systematic. Shelling continues, in fact, along the entire border, with an intensity of 200-400 explosions per day... The intensity of enemy sabotage groups has increased," he said.

(Reporting by Yuliia Dysa, Anastasiia Malenko; Writing by Tom Balmforth; Editing by Sharon Singleton)