Russian President Vladimir Putin may finalise his plan to annex four Ukrainian regions later today, even as his forces are being pushed back by Ukraine on two separate battlefield fronts, shrinking the amount of seized territory he controls.

Russia, which has escalated its seven-month war with its annexation drive, a military mobilisation and warnings of nuclear weapons use, looks in a hurry to complete a process Ukraine and the West say is illegal and will not be recognised.

Mr Putin is expected to sign a law this evening formally incorporating the four regions, representing about 18% of Ukraine's territory, into Russia. The two chambers of Russia's parliament have already ratified the legislation.

In Brussels, the European Union summoned Russia's envoy to the EU to reject Moscow's "illegal annexation" and urge it to unconditionally withdraw all of its troops from the entire territory of Ukraine.

Russia does not completely control any of the four regions it says it is annexing - Donetsk, Luhansk, Zaporizhzhia and Kherson - and the Kremlin has said it has yet to determine the final borders of the annexed territory.

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Russian forces in the eastern Donetsk and southern Kherson regions have been forced to retreat in recent days and appear to be struggling to halt an increasingly Western-equipped Ukrainian army.

US President Joe Biden told Ukraine's leader Volodymyr Zelensky in a call that the United States would provide Kyiv with $625 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, the White House said.

Earlier, the European Union also rejected Moscow's "illegal annexation" and urged it to unconditionally withdraw all of its troops from the entire territory of Ukraine.

Read more: Latest Ukraine stories

Russia is hoping a "partial mobilisation" it announced two weeks ago could help turn the tide.

Defence Minister Sergei Shoigu was cited by the RIA news agency as saying that Russia had so far called up more than 200,000 reservists out of a planned 300,000 men.

Many Russian men have fled the country rather than fight in Ukraine, however, and Russian lawyers say they are working flat out to offer advice to men who want to avoid being drafted. Some Russians are making journeys of thousands of miles by car, train and plane to escape.

Southern breakthrough

President Alexander Lukashenko of Belarus, Ukraine's northern neighbour and an ally of Mr Putin, accused Kyiv of sending 15,000 troops to the border area to build defences and conduct reconnaissance, actions he called "provocations".

In their biggest breakthrough in the south since the seven-month-old war began, Ukrainian forces retook several villages in an advance along the strategic Dnipro River yesterday, Ukrainian officials and a Russian-installed leader in the area said.

Ukrainian forces in the south destroyed 31 Russian tanks and one multiple rocket launcher, the military's southern operational command said in an overnight update.

Ukrainian video footage from Novopetrivka, a newly recaptured village in the Kherson region, showed Ukrainian soldiers removing a Russian flag from a power pylon, wiping their feet on it and setting it alight. They then raised a Ukrainian flag.

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, hours after Mr Putin proclaimed the annexation of the province last week.

Russian defence ministry maps presented today appear to show rapid withdrawals of Russian invasion forces from areas in eastern and southern Ukraine where they have been under severe pressure from the Ukrainian counteroffensive.

An aerial view of destroyed buildings in Izium

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.

Ukraine has released little information about Kherson, in keeping with a policy of withholding details about ongoing advances.

Mr Zelensky said in his nightly video address that Ukraine's army had seized back towns in a number of areas, without giving details.

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.

In the early hours of this morning, a Russian missile crashed into the outskirts of the Ukraine-controlled eastern city of Kramatorsk. A Reuters reporter on the scene said the missile had gouged a huge crater in the backyard of a house.

Destruction is seen in a civilian neighbourhood in Siversk city subdued to heavy shelling

In a decree, Mr Zelensky has formally declared any talks with Mr Putin "impossible", while leaving the door open to talks with Russia 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".

Kremlin spokesman Dmitry Peskov also said Moscow did not want to take part in "nuclear rhetoric" spread by the West, after Britain's Times newspaper reported that NATO had warned members Mr Putin might test an atomic weapon on Ukraine's border.

A Western diplomat told Reuters that NATO had not warned its members of a Russian nuclear threat.

Separately, a NATO official said the alliance had not observed any changes in Russia's nuclear posture but that NATO remained vigilant.