Three bridges over Seym River in Russia now destroyed by Ukraine

Ukraine has destroyed a third bridge over the Seym River in the Kursk region, as part of an apparent attempt to expand what President Volodymyr Zelenskiy has described as a military “buffer zone” inside Russia.

Related: Kursk incursion aimed at creating buffer zone to protect Ukraine, Zelenskiy says

According to Russian officials, the bridge in the village of Karyzh was damaged overnight by targeted Ukrainian “shelling”. It was the last major crossing on this part of the front, following the destruction on Friday and Saturday of two bridges further east over the same river.

Ukraine’s armed forces are now poised to push forward from their existing bridgehead around the Russian town of Sudzha, captured two weeks ago during a surprise offensive . They are seeking to encircle Russian troops – some of them conscripts – who are stuck south of the river in the Korenevsky district.

If the operation succeeds, Ukraine will gain another 700 sq km (270 sq miles) of Russian land. Russia has built pontoon bridges across the river in order to supply its forces, but these are vulnerable to close-range Ukrainian strikes from US-supplied Himars systems, truck-mounted mobile rocket launchers.

On Monday Kyiv captured two more Russian villages, Snagost and Apanasovka . The pace of its advance into Kursk oblast has slowed in recent days, however. The Kremlin has scrambled reserves to try to stop Ukrainian combat units from advancing further.

“The situation is messy there,” a senior Ukrainian official told the Guardian. “The Russians have pulled in extra troops. Some are capable, some are not. The Russians have found it extremely difficult to recapture lost territory.”

Ukraine’s commander-in-chief, Oleksandr Syrskyi, was waging a “high-speed” campaign, with his troops constantly on the move, the official said. Syrskyi wanted to avoid a dug-in “positional war”, similar to the one taking place in eastern Ukraine, the person said, adding: “It’s risky. But the narrative of the war has changed. Everything is possible.”

While Ukrainian forces have made quick progress around Sudzha, Russian troops have been steadily making gains in the east of Ukraine. On Monday they captured the town of Niu-York, raising the Russian flag and renaming the town Novgorodske, Kremlin military bloggers reported.

For months, Russian forces have also been edging closer to the Ukrainian city of Pokrovsk, gobbling up surrounding villages. Ukrainian defenders have retreated in the face of airstrikes on their positions, followed by round-the-clock infantry assaults.

Related: ‘Soon there will be just death’: Russian shells flay away at hope in Ukraine’s frontline cities

On Monday Pokrovsk’s military administrator, Serhii Dobryak, said fighting was likely to engulf the city in less than two weeks. He urged residents to pack up and leave. About 60% had already gone, he said. He added that families with children would soon be forced to evacuate under emergency rules.

In neighbouring Myrnohrad – now just a few kilometres from the frontline – fewer than 16,000 people remained. Banks, pharmacies, shops and markets were closing, together with all organisations and institutions. The city hospital was shutting down as well, with the exception of a few doctors who would treat the wounded.

The imminent Russian attack on Pokrovsk will complicate Ukraine’s attempts to defend its Donbas region, where war has raged since 2014. The Russians are close to capturing a highway that links Pokrovsk to a string of major cities to the north, including the garrison settlements of Kramatorsk and Sloviansk.

The Kursk raid was conceived in part as a way to relieve pressure on Pokrovsk and the city of Toretsk, also under intense Russian fire. So far, though, the Kremlin has transferred irregular forces from the rear, as well as units based in the occupied south of Ukraine. If anything, it has thrown more resources into the battle for Pokrovsk.

Speaking on Sunday, Zelenskiy said Ukraine’s military incursion into Russia was carried out to create a permanent buffer zone. It would prevent further attacks by Moscow across the border, he suggested, after a Russian offensive in May against the Ukrainian city of Vovchansk. Fighting there continues.

Previously Zelenskiy said the incursion would protect communities in Ukraine’s bordering Sumy region from constant shelling. “It is now our primary task in defensive operations overall to destroy as much Russian war potential as possible and conduct maximum counteroffensive actions. This includes creating a buffer zone on the aggressor’s territory – our operation in the Kursk region,” he declared.

Zelenskiy urged Ukraine’s international partners to speed up deliveries of weapons. He also called on the US, UK and France to take crucial “decisions” – a plea for restrictions to be lifted on the use of long-range western weapons against strategic airbases and other military targets within Russia.

A spokesperson for Keir Starmer said British backing for Kyiv was unwavering. “The prime minister remains absolutely resolute in his support for Ukraine,” they said, adding that the ban on the use of British Storm Shadow missiles inside Russia remained.

Zelenskiy’s latest comments suggest Ukraine will seek to hold on to its gains in Kursk oblast, ahead of possible negotiations to end the war. On Monday Russia’s presidential aide Yuri Ushakov made clear Moscow was not ready for peace discussions because of what he called Ukraine’s Kursk “adventure”. “We will not talk,” he said.