Russia was launching “constant shelling” against Kharkiv Saturday, according to a readout from the National Police. Ukrainian President Volodymyr Zelensky has had two briefings from his Commander-in-Chief, Alexander Syrsky, on Saturday.

Russia attacked with airstrikes in Vilcha, Liptsy, Liman, Izbitskoye, Veseloye, Petrovka, Kazachya Lopan, Sinelnikovo, Volchansk in Kharkiv, according to an assessment from the General Staff of the Armed Forces of Ukraine.

The situation is “incredibly difficult,” the head of the Vovchansk military administration, Tamaz Hambarashvili, said, according to the AP.

Russia has primarily focused its war effort on Donetsk in eastern Ukraine, with only marginal gains. But in recent weeks, Russia has been focusing attacks on energy infrastructure in what may have been an attempt to prepare for further assault, the AP reported.

The timing is sensitive for Ukraine, which has been suffering from a thinly stretched military and resources in recent months. The focus of the attack is likely aimed at bogging Ukrainian resources down in Kharkiv to dilute Ukraine’s focus on thwarting Russian attacks in eastern Ukraine, analysts at the Institute for the Study of War said. So far, the operation had given Russia “tactically significant gains,” the ISW said.

Russia claimed it had captured five villages between Kharkiv and Russia.

Russia began its surprise offensive in the Vovchansk community on Friday three miles from the Russian border, killing at least two civilians, Oleg Sinegubov, the head of the regional administration, said in a post on Telegram.

Ukraine had won Vovchansk back from Russian occupation early in the war.

Nearly 1,000 residents had evacuated Kharkiv between Friday and Saturday, according to figures from the National Police

According to AP journalists on the ground, buildings were destroyed and others were on fire Saturday. Sky News reported smoke billowing out from buildings.

Ukraine was working to strengthen its positions in Kharkiv, Zelensky said in an address on Saturday.

“Our defense operations continue in the Kharkiv region,” Zelensky said. “Disrupting Russian offensive plans is now the number one task. Accomplishing this task depends on every soldier, every sergeant, every officer.”

The onslaught comes weeks after Russian President Vladimir Putin hinted that he may seek to establish what he called a “buffer” zone between Russia and Ukraine—but on only Ukrainian territory. Analysts at the time believed that his statements could be laying the groundwork to force a zone on Ukrainian territory that is painted like a peaceful buffer between Ukraine and Russia, when in actuality it may be a cover for forced annexation of Ukrainian territory by another name.

The operation is also likely aimed at pushing Ukrainian forces back from Belgorod in Russia as well, according to the ISW.

“Russian forces will likely leverage their tactical foothold in northern Kharkiv Oblast in the coming days to intensify offensive operations and pursue the initial phase of an offensive effort likely intended to push back Ukrainian forces from the border with Belgorod Oblast,” ISW analysts said.

Ukrainian forces have been waging attacks for months against Belgorod that have put Russia on the backfoot, evacuating residents as Ukraine takes the fight to Russian soil.