Ukraine has launched a surprise counterattack in the north-east Kharkiv region, stretching Russian forces who are also facing Ukrainian attacks in the south.

An official representing the Russian-controlled Donetsk People’s Republic said on Tuesday that Ukrainian forces “encircled” Balakliia, an eastern town of 27,000 people situated between Kharkiv and Russian-occupied Izium.

“Today, the Ukrainian armed forces, after prolonged artillery preparation … began an attack on Balakliia,” Daniil Bezsonov said on Telegram.

“At this time, Balakliia is in operative encirclement and within the firing range of Ukrainian artillery. All approaches are cut off by fire,” he said, adding that a successful Ukrainian offensive would threaten Russian forces in Izium, a strategically important town that Russia has been using for its own offensive in eastern Ukraine.

Unverified footage circulating on social media on Wednesday showed what looked like a Ukrainian soldier posing in front of an entrance sign for Balakliia.

Analysts have said that the initial target of the offensive could be the city of Kupyansk, a key road hub for Russian supplies heading south from the border into eastern Ukraine. Ukraine’s army has yet to comment on the alleged offensive.

One of Ukrainian President Volodymyr Zelenskiy’s advisers, Oleksiy Arestovych, said on Tuesday night that “lightning-fast changes are taking place” in the Kharkiv region, in parallel to the southern offensive in the Kherson region announced by Ukraine’s military last week.

The Institute for the Study of War (ISW), a Washington-based thinktank, reported on Wednesday that Ukrainian forces probably captured Verbivka (less than three kilometres north-west of Balakliia) on Tuesday, citing geo-locatable images posted by Ukrainian soldiers.

Without specifying any locations, the Luhansk regional governor, Serhiy Haidai, told Ukrainian television on Wednesday that a “counterattack is under way and…our forces are enjoying some success. Let’s leave it at that.”

Ukraine’s military did give some confirmation of the alleged offensive on Wednesday claiming Ukraine’s army shot down a Russian military Sukhoi Su-25 aircraft near Volokhiv Yar, a town about 12 miles from Balakliia.

On Tuesday afternoon, Serhiy Leshchenko, an adviser to Zelenskiy, published and then later deleted a tweet saying that there would be “great news from President Zelenskiy on the counteroffensive operation in Kharkiv region”. But in his nightly address, Zelenskiy did not mention the Kharkiv offensive and instead urged people to pay equal attention to fighting on all areas of the front.

“It is simply unfair when there are a lot of talks about certain areas of the front, while others seem to be forgotten,” he said.

Ukraine for a long time spoke openly of its intent to launch a large-scale southern offensive in the Kherson region, forcing Russia to transfer some of its most elite units from the east to Kherson.

According to the ISW, this has opened up opportunities for Ukraine to launch attacks near Kharkiv.

“Russia’s deployment of forces from Kharkiv and eastern Ukraine to Ukraine’s south is likely enabling Ukrainian counterattacks of opportunity,” the thinktank wrote late on Tuesday.

“The 6 September Ukrainian counterattack in Kharkiv was likely an opportunistic effort enabled by the redeployment of Russian forces away from the area to reinforce Russian positions against the Ukrainian counteroffensive in Kherson oblast,” ISW added.

Rob Lee, a military analyst at the Foreign Policy Research Institute, said the Ukrainian attack indicated that Russian forces near Kharkiv were stretched.

“Russia doesn’t have strong reserves in this area that could be sent quickly to plug gaps and reinforce key towns. Ukraine may have a numerical and armour superiority here.”

The Russian government and its defence ministry have also not commented on Ukraine’s alleged offensives.

Speaking at an economic forum in the far-eastern Russian city of Vladivostok on Wednesday, Russian President Vladimir Putin, said Russia had not lost anything as a result of its military campaign in Ukraine.

A number of prominent pro-Kremlin bloggers volleyed criticism at the Russian authorities after Ukraine’s latest offensive. Igor Girkin, a Russian ultra-nationalist and former military leader for the Russian-backed republics in eastern Ukraine, said the outskirts of Balakliia were defended by poorly equipped mobilised forces from the Donbas region who “didn’t know how to use available heavy weapons”.

He added that Ukraine also took control of the settlement of Volokhiv Yar overnight.

Aleksandr Kots, a pro-Kremlin war blogger, on Wednesday slammed the Russian authorities for hiding “bad news. We need to start doing something about the system where our leadership doesn’t like to talk about bad news and their subordinates don’t want to upset their bosses,” he said.

• An AP report