Ukrainian forces are advancing in both the north-east and south of the country in further setbacks to Vladimir Putin, British defence chiefs said on Wednesday.
They were also closing in on a provincial border of territory which Putin claimed was Russian after he annexed it in a land grab condemned by world leaders.
Their advance would put more pressure on the Russian president as his invasion flounders.
In its latest intelligence update, the Ministry of Defence in London said: “Ukraine continues to make progress in offensive operations along both the north-eastern and southern fronts.
“In the north-east, in Kharkiv Oblast (province), Ukraine has now consolidated a substantial area of territory east of the Oskil River.”
The illegal and unprovoked invasion of Ukraine is continuing.
The map below is the latest Defence Intelligence update on the situation in Ukraine – 05 October 2022
Find out more about the UK government's response: https://t.co/5DOyzVlTwJ
🇺🇦 #StandWithUkraine 🇺🇦 pic.twitter.com/adOQRJTszG
— Ministry of Defence 🇬🇧 (@DefenceHQ) October 5, 2022
The briefing continued: “Ukrainian formations have advanced up to 20 km (12.5 miles) beyond the river into Russia’s defensive zone towards the supply node of the town of Svatove.
“It is highly likely that Ukraine can now strike the key Svatove-Kremina road with most of its artillery systems, further straining Russia’s ability to resupply its units in the east.
“Politically, Russian leaders will highly likely be concerned that leading Ukrainian units are now approaching the borders of Luhansk Oblast (province), which Russia claimed to have formally annexed last Friday.”
In Ukraine, president Volodymyr Zelensky said his military had made major, rapid advances against Russian forces in the past week, taking back dozens of towns in regions in the south and east that Mr Putin has declared annexed.
“This week alone, since the Russian pseudo-referendum, dozens of population centres have been liberated. These are in Kherson, Kharkiv, Luhansk and Donetsk regions all together,” he said in a Tuesday night address.
Russia moved to annex those regions after holding what it called referendums over several days from September 23 – votes that were denounced by Kyiv and Western governments as illegal and coercive.
Mr Putin proclaimed their annexation last Friday.
However, Mr Zelensky named eight small towns in Kherson in the south as recently having been recaptured.
A video released by the Ukraine defence ministry appeared to show the Ukrainian flag being raised over one of those communities, Davydiv Brid, in Kherson.
Ukrainian forces retook several villages in an advance along the strategic Dnipro River on Monday, Ukrainian officials and a Russian-backed leader in the area said.
In the east, Ukrainian forces have been expanding an offensive after capturing the main Russian bastion in the north of Donetsk, the town of Lyman.
Russian forces in the Donetsk and Kherson regions have been forced to retreat in recent days and appear to be struggling to halt an increasingly Western-equipped Ukrainian army.
“In some areas of the front line it was possible to extend the area we hold from between 10 to 20 km,” the southern Operational Command of the Ukrainian Armed Forces said on Wednesday.
Russian forces were destroying their reserves of ammunition and trying to destroy bridges and crossings in order to slow the Ukrainian advance, the UAF said in its daily report.
In Kherson, withdrawing Russian forces were planting mines on “infrastructure facilities” and in homes, it said.
In the past 24 hours, Russia had lost 31 servicemen, more than 40 pieces of equipment, including eight tanks, 26 armoured vehicles, and a large calibre howitzer, it said.
Mr Putin on Wednesday signed a law formally annexing four partly Russian-controlled Ukrainian regions, state-owned news agency TASS reported.
They represent about 18 per cent of Ukraine’s territory.
But Russia does not fully control any of the four regions it claims - Donetsk and Luhansk in eastern Ukraine and Zaporizhzhia and Kherson in the south - and the Kremlin has said it had yet to determine the borders of the annexed territory.
Mr Putin has escalated his seven-month war with the annexation drive, a military mobilisation and warnings of a possible recourse to nuclear weapons to protect its territory.
Moscow hopes a “partial mobilisation” it announced two weeks ago can help reverse a series of battlefield setbacks.
Defence Minister Sergei Shoigu was cited by the RIA news agency on Tuesday as saying that Russia had so far called up more than 200,000 reservists out of a planned 300,000 men.
Tens of thousands of Russian men have fled the country rather than fight in Ukraine, however, and Russian lawyers say they are working flat out to advise men who want to avoid being drafted.
Russian defence ministry maps presented on Tuesday appeared to show rapid withdrawals of Russian forces from areas in eastern and southern Ukraine where they have been under severe pressure from the Ukrainian counteroffensive.
The ministry’s daily video briefing made no mention of any pullbacks, but on maps used to show the location of purported Russian strikes, the shaded area designating Russian military control was much smaller than the day before.
On the eastern front, Denis Pushilin, the Russia-backed leader in Donetsk, said Russian forces were building a serious line of defence around the city of Kreminna after being pushed back.
US President Joe Biden told Mr Zelensky in a call that the United States would provide Ukraine with $625 million (£545 million) in new security assistance, including High Mobility Artillery Rocket System (HIMARS) launchers.
He also reiterated that the United States would “never recognise” Russia’s annexation of Ukrainian territory.
Earlier, the European Union also rejected Moscow’s “illegal annexation” and urged it to unconditionally withdraw its troops.
Russia’s ambassador to the United States, Anatoly Antonov, said in a social media post that the US decision to send more military aid to Ukraine posed a threat to Russia’s interests and increased the risk of a military clash between Russia and the West.
The UN General Assembly is expected to vote next week on a draft resolution denouncing Russia’s action, diplomats said. Russia vetoed a similar resolution in the 15-member Security Council last week.
In a decree on Tuesday, Mr Zelensky formally declared any talks with Mr Putin “impossible”, while leaving the door open to talks with Moscow if it got a new leader.
The Kremlin said that what it calls its “special military operation” in Ukraine would not end if Kyiv ruled out talks, adding that it “takes two sides to negotiate”.