(CNN) — Ukraine’s top general says the situation in the northern Kharkiv region has “significantly worsened” after Russia claimed to have captured four further villages as it expanded its surprise cross-border offensive.

A Ukrainian regional official insisted Russia’s progress was not yet “significant” but admitted ground fighting in the area was spreading. Meanwhile, speaking on British television, UK Foreign Secretary David Cameron acknowledged it was an “extremely dangerous moment,” adding that Russia had effectively “invaded [Ukraine] again.”

The precise goal of Russia’s new push – which began in the early hours of Friday morning – is unclear. It may be to create a buffer zone designed to reduce Ukrainian attacks on Russian territory, or possibly even a renewed assault on the city of Kharkiv, 30 kilometers (18 miles) to the south.

Equally, it could be an attempt to draw Ukrainian forces away from other key Russian objectives further south – which was the rationale Ukrainian President Volodymyr Zelensky offered in his Sunday evening address.

“The idea behind the attacks in Kharkiv region is to stretch our forces and undermine morale and the motivational foundations of Ukrainians’ ability to defend ourselves,” he said.

According to information from the Defense Ministry in Moscow, Russia now claims control over nine Ukrainian border villages, which are clustered in two areas, now just a few dozen kilometers apart, to the northeast of Kharkiv city.

A Ukrainian military blogger, Myroshnykov, said Sunday afternoon that Russian troops had now pushed south and entered the town of Vovchansk, though the pace of advance there had slowed in the face of Ukraine’s defending forces.

Myroshnykov went on to predict an attempt by Russia to make its numerical advantage count and throw large numbers of infantry soldiers into the battle.

“They will try to seize the area with a lot of cannon fodder,” he wrote on his Telegram channel, referencing previous Russian offensives which reports suggest have resulted in many Russian troops killed.

A Ukrainian military spokesman later denied reports Russian forces were inside Vovchansk, saying Ukraine’s armed forces had succeeded in keeping them at bay. Ahead of any stepped-up Russian actions, Ukrainian officials have been evacuating hundreds of local residents to safety.

An idea of the destruction being wrought on the town and surrounding villages came from regional Ukrainian leader, Oleh Syniehubov, who said Saturday that Russia had dropped more than 20 huge glide bombs on the area in under two days.

These bombs have a minimum weight of 250 kgs, but can go up to 1,500 kgs, and are among the most destructive mass-produced devices currently used by Russia in the war. They have previously been used as a precursor to a major attempt to seize ground.

Oleksandr Syrskyi, Ukraine’s army chief, told readers of his social media channels on Sunday that defensive operations were ongoing and that battlefield conditions were difficult.

“This week, the situation in Kharkiv Oblast has significantly worsened. Currently, there are ongoing battles in the border areas along the state border with the Russian Federation,” he wrote on Telegram.

Syniehubov, the head of the Kharkiv Regional Military Administration, insisted control over the border villages was still contested rather than ceded, however the influential DeepStateMap group – a Ukrainian organisation monitoring developments on the battlefield – showed seven Ukrainian villages under either full or partial Russian control.

So far, Russian gains are not sufficient to put its big artillery pieces within range of Kharkiv city, but residents there admit the situation is frightening.

One woman, Anna Ivanova, speaking to Reuters, said: “Of course it is scary, we keep on monitoring the situation, following the news but we remain home and are not planning to go anywhere. All depends on what happens.”

Another Kharkiv resident, Olena Pidhirna, said: “Despite all those attacks, air raid alarms, we live normal lives, everyone keeps going out, playing, going to school, enjoying themselves, working. Life goes on, as normal.”

It is not just in Ukraine’s north that Kyiv’s armed forces are losing ground.

DeepStateMap also shows Russia making further gains in two other key areas, most notably towards the town of Chasiv Yar, which has become an important forward military position, and further south in the industrial town of Krasnohorivka, where Russian troops are now in control of the town’s main facility, a brick factory.

Meanwhile, Russian authorities say eight people are known to have died in an explosion in a residential building in Belgorod. CCTV video of the blast shows a huge plume of dark grey smoke emerging from one of the building’s lower floors, before part of the building collapses on itself.

Russian television later broadcast video of rescuers at work looking for survivors among the rubble. Russian officials said the blast was caused by a Ukrainian missile attack.

And in a separate development, a fire briefly broke out at the Volgograd oil refinery, the largest oil refinery in southern Russia, after a Ukrainian drone attack overnight into Sunday, according to the region’s governor.

“On the night of May 12, air defense and electronic warfare forces repelled a UAV attack on the territory of the Volgograd region,” Andrey Bocharov said in a post on Telegram.

“As a result of the consequences of a UAV crash with subsequent detonation, a fire broke out at the Volgograd Refinery site,” he said.

The fire has been extinguished and there were no casualties, he said.

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