Russian forces closed in on the frontline town of Avdiivka in east Ukraine on Tuesday, as Kyiv warned Moscow was stepping up strikes in an attempt to encircle it completely.
The Russian attacks come after months of a slower than expected Ukrainian counter-offensive, launched this summer to claw back territory lost during Moscow's invasion.
Avdiivka is an industrial town that had a pre-war population of around 31,000. An estimated 2,000 remain.
It is symbolically and strategically important to Kyiv, lying just north of the Moscow-controlled city of Donetsk that was seized by separatist forces in 2014.
Avdiivka has since become a symbol of Ukrainian resistance against Moscow, with Kyiv clinging on there despite relentless Russian attacks during the Kremlin's almost 20-month long offensive.
Local officials told AFP that Moscow's forces launched an artillery attack on the town in the morning and were firing incessantly.
"For over a year, there has been the danger that (Avdiivka) can be occupied, but now the situation has worsened rapidly," said Vitaliy Barabash, head of the town's administration.
He said Russian forces were trying to besiege the eastern town using "all means."
"Our Avdiivka is under massive attacks by Russian artillery and aviation," presidential aide Andriy Yermak later said on social media.
The Ukrainian army said in a briefing that Russia had "intensified offensive actions in Avdiivka" and that Moscow's forces had deployed tanks and armoured vehicles.
"Our defenders repelled all enemy attacks and prevented the loss of lines and positions," it said.
Russian forces control territory to the south, east and north of Avdiivka.
- Zelensky in Romania -
The attacks came as Ukrainian President Volodymyr Zelensky held talks on grain and security with Romanian counterpart Klaus Iohannis, after Moscow struck areas of Ukraine close to the Romanian border.
On his first visit to the NATO country since Russia invaded Ukraine in February last year, Zelensky said the strikes near the border were "a threat, not only to our state".
Since exiting a deal allowing safe grain shipments via the Black Sea, Moscow has ramped up strikes on Ukraine's southern regions, including Danube river ports near Romania.
Last month, Romania found drone debris several times near its border with Ukraine, prompting authorities to build air raid shelters for one border village.
Zelensky and Iohannis discussed how to increase Kyiv's grain exports being shipped out via Romania, with Russia's invasion of Ukraine hindering the country's access to the Black Sea.
"A grain corridor from Ukraine through Moldova to Romania will soon be operational," Zelensky announced.
The pair said they signed a bilateral declaration on security and transport cooperation.
During a press conference, Zelensky also said Ukrainian pilots would train at an F-16 pilot training centre to be created in Romania.
- Rift between Moscow, allies -
President Vladimir Putin meanwhile prepared to leave Russia for the first time this week since the International Criminal Court (ICC) issued an arrest warrant for him in March.
The ICC issued the warrant for Putin for the war crime of illegally deporting Ukrainian children to Russia, with its members expected to make the arrest should he step foot on their territory.
The Russian leader is due to travel to Kyrgyzstan -- which is not a member of the ICC -- for a summit of a Moscow-led alliance later this week.
But rifts among Moscow's ex-Soviet allies have been emerging in recent months, with many rattled by the Ukraine invasion.
Bishkek announced that Armenian leader Nikol Pashinyan would skip the summit, amid a growing rift between Yerevan and Moscow.
Pashinyan's decision to snub the meeting of the Moscow-led regional grouping is a further blow to ties between Yerevan and Moscow, which have soured in recent weeks.
Pashinyan has criticised Moscow's role in the Karabakh crisis, with Russia unwilling to intervene when Azerbaijan launched a lightning operation to regain control of the breakaway Nagorno-Karabakh region.
Pashinyan visited a European summit last week after Armenian lawmakers moved to join the ICC, angering Moscow.
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