KYIV, Ukraine (AP) — Western defense officials and analysts on Saturday said they believed the Russian forces were setting up a new defensive line in Ukraine’s northeast after Kyiv’s troops broke through the previous one and tried to press their advances further into the east.

The British Defense Ministry said in a daily intelligence briefing that the line likely is between the Oskil River and Svatove, some 150 kilometres (90 miles) southeast of Kharkiv, Ukraine’s second-largest city.

The new line comes after a Ukrainian counteroffensive punched a hole through the previous front line in the war and recaptured large swaths of land in the northeastern Kharkiv region that borders Russia.

Moscow “likely sees maintaining control of this zone as important because it is transited by one of the few main resupply routes Russia still controls from the Belgorod region of Russia,” the British military said, adding that ”a stubborn defense of this area” was likely, but that it remained unclear whether the Russians would be able to withstand another concerted Ukrainian assault.

Ukrainian forces, in the meantime, continue to cross the key Oskil River in the Kharkiv region as they try to press on in a counteroffensive targeting Russian-occupied territory, according to the Washington-based Institute for the Study of War.

The Institute said in its Saturday report that satellite imagery it examined suggest that Ukrainian forces have crossed over to the east bank of the Oskil in Kupiansk, placing artillery there. The river, which flows south from Russia into Ukraine, had been a natural break in the newly emerged front lines since Ukraine launched its push about a week ago.

“Russian forces are likely too weak to prevent further Ukrainian advances along the entire Oskil River if Ukrainian forces choose to resume offensive operations,” the institute said.

After the Russian troops retreated from the city of Izium, Ukrainian authorities discovered a mass grave site, one of the largest so far discovered. President Volodymyr Zelenskyy said Friday that more than 440 graves have been found at the site but that the number of victims is not yet known.

Zelenskyy said the graves contained the bodies of hundreds of civilian adults and children, as well as soldiers, and some had been tortured, shot or killed by artillery shelling. He cited evidence of atrocities, such as a body with a rope around its neck and broken arms.

Videos circulating online on Saturday indicated that Ukrainian forces are also continuing to take land in the country’s embattled east.

One video showed a Ukrainian soldier walking past a building, its roof destroyed, then pointing up over his shoulder at a colleague hanging the blue-and-yellow Ukrainian flag over a mobile phone tower. The soldier in the video identified the seized village as Dibrova, just northeast of the city of Sloviansk and southeast of the embattled city of Lyman in Ukraine’s Donetsk region.

Another online video showed two Ukrainian soldiers in what appeared to be a bell tower. A Ukrainian flag hung as a soldier said they had taken the village of Shchurove, just northeast of Sloviansk.

The Ukrainian military and the Russians did not immediately acknowledge the change of hands of the two villages.

Elsewhere in Ukraine, Russian forces continued to pound cities and villages with missile strikes and shelling.

A Russian missile attack early Saturday started a fire in Kharkiv’s industrial area, said Oleh Syniehubov, the regional governor. Firefighters extinguished the blaze.

Syniehubov said remnants of the missiles suggest the Russians fired S-300 surface-to-air missiles at the city. The S-300 is designed for striking missiles or aircraft in the sky, not targets on the ground. Analysts say Russia’s use of the missiles for ground attacks suggest they may be running out of some precision munitions as the monthslong war continues.

In the southern Zaporizhzhia region, a large part of which is occupied by the Russians, one person was wounded after the Russian forces shelled the city of Orikhiv, Zaporizhzhia’s Ukrainian governor Oleksandr Starukh reported on Telegram. Starukh said the Russian troops also shelled two villages in the region, destroying several civilian facilities there.

The central Dnipropetrovsk region also came under fire overnight, according to its governor, Valentyn Reznichenko. “The enemy attacked six times and launched more than 90 deadly projectiles on peaceful cities and villages,” Reznichenko said.

Meanwhile, Ukraine’s atomic energy operator, Energoatom, said a convoy of 25 trucks has brought diesel fuel and other critical supplies to the Zaporizhzhia nuclear power plant — Europe’s largest, which was shut down a week ago amid fears that fighting in the area could result in a radiation disaster.

The trucks were allowed through Russian checkpoints on Friday to deliver spare parts for repairs of damaged power lines, chemicals for the operation of the plant and additional fuel for backup diesel generators, Energoatom said in a statement.

The six-reactor plant was captured by Russian forces in March but is still operated by Ukrainian engineers. Its last reactor was switched off Sunday after repeated power failures due to shelling put crucial safety systems at risk.