Russian forces on Wednesday pressed closer to the centre of an industrial city in a drive to grab a swathe of eastern Ukraine.

Ukraine’s General Staff said Russian forces, now 98 days into their invasion, were pounding infrastructure in eastern and southern regions including the symbolically important city of Sievierodonetsk, which they entered on May 27. It has been the main focus of their ground offensive for several weeks.

Sievierodonetsk, a Soviet-era city, houses a large chemical factory. According to local governor Serhiy Gaidai, a Russian airstrike hit the plant on Tuesday, blowing up a tank of toxic nitric acid and releasing a plume of pink smoke.

Russia “attacked the Azot factory from a plane, resulting in the release of toxic substances”, Gaidai said, urging residents to remain inside. Reuters could not independently confirm the cause of the incident.

According to the Russian defence ministry, Russia’s nuclear forces were holding drills in the Ivanovo province, northeast of Moscow, the Interfax news agency reported.

Some 1,000 servicemen were exercising in intense manoeuvres using more than 100 vehicles including Yars intercontinental ballistic missile launchers, it cited the ministry as saying. The report did not mention the US decision.

Russia has also completed testing of its hypersonic Zircon cruise missile and will deploy it by the end of the year on a new frigate of its Northern Fleet, a senior military officer said on Wednesday.

Ukraine’s General Staff said Russian forces continued to pound northern, southern and eastern districts of Sievierodonetsk.

If Russia captures the city and its smaller twin Lysychansk on the west bank of the Siverskyi Donets river, it will hold all of Luhansk, one of two provinces in the eastern Donbas region that Moscow claims on behalf of separatists and a key war aim of President Vladimir Putin.

Ukrainian forces now control just 20 per cent of Sievierodonetsk, Russian forces 60 per cent and the rest has become a “no-man’s land”.