The US and Germany pledged to send Ukraine some of the most advanced weapons yet as Russian forces waging a grinding offensive in the east closed in on capturing a key city.

Germany said on Wednesday it will supply Ukraine with modern anti-aircraft missiles and radar systems, and the US planned to unveil a new weapons package later in the day that includes high-tech, medium-range rocket systems.

The US is trying to strike a balance by helping Ukraine battle ferocious Russian artillery barrages while not providing weapons that could allow Ukraine to hit targets deep inside Russia and trigger an escalation in the war.

The Kremlin accused the US of “pouring fuel on the fire”.

Western arms have been critical to Ukraine’s success in stymieing Russia’s much larger and better-equipped military, thwarting its effort to storm the capital and forcing Moscow to shift its focus to the industrial Donbas region in the east.

But as Russia bombards towns in its inching advance in the east, Ukrainian President Volodymyr Zelensky has repeatedly pleaded for more and better weapons and accused the West of moving too slowly.

Military analysts say Russia is hoping to overrun the Donbas before any weapons that might turn the tide arrive.

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The Biden administration is expected to announce a new weapons package that will include high-tech, medium-range rocket systems (Evan Vucci/AP)

German Chancellor Olaf Scholz told politicians that the surface-to-air IRIS-T SLM missiles it will send are the most modern air defence system the country has. They can operate at longer ranges than the Cold War-era anti-aircraft equipment it previously provided.

“With this, we will enable Ukraine to defend an entire city from Russian air attacks,” he said. The radar systems Germany is sending will help Ukraine locate enemy artillery.

In addition to the rocket systems, the US package will include helicopters, Javelin anti-tank weapons, tactical vehicles, spare parts and more, two senior administration officials said. The officials spoke on condition of anonymity to preview the package before it was formally unveiled.

One official noted that the advanced rocket systems will give Ukrainian forces greater precision in targeting Russian forces inside Ukraine.

The US has received assurances that Ukraine will not fire rockets into Russian territory, according to the senior administration officials. Medium-range rockets can generally can travel about 45 miles.

Kremlin spokesman Dmitry Peskov said Moscow does not trust such assurances.

“We believe that the US is deliberately and diligently pouring fuel on the fire,” he said.

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Ukrainian servicemen prepare unexploded Russian ammunition for destruction in the outskirts of Kyiv (Natacha Pisarenko/AP)

The announcements came as a regional governor said Russian forces now control 70% of Sievierodonetsk, a city that is key to Moscow’s efforts to complete its capture of the Donbas, where Ukrainian forces and Russian-backed separatists have fought for years and where the separatists held swaths of territory even before the invasion.

Luhansk Governor Serhiy Haidai said Ukrainian and Russian forces were battling it out in the streets.

The only other city in Luhansk that the Russians have not yet captured, Lysychansk, is still fully under Ukrainian control, he said, but is likely to be next.

“If the Russians manage to take full control over Sievierodonetsk within two to three days, they will start installing artillery and mortars and will shell Lysychansk more intensively,” Mr Haidai said.

Mr Zelensky, meanwhile, said the country is losing between 60 and 100 soldiers a day in the fighting.

In southern Ukraine, a regional governor sounded a more positive note, saying Russian troops are retreating and blowing up bridges behind them.

“They are afraid of a counterattack by the Ukrainian army,” Vitaliy Kim, governor of the Mykolayiv region, said on the Telegram messaging app.