Ukraine has urged its allies to send more weapons as its forces dug in to slow Russia’s military advance through the eastern Donbas region while Russian President Vladimir Putin warned the US and European countries of consequences if they implemented further sanctions.

Signalling that the Kremlin was in no mood for compromise, Putin said continued use of sanctions against Russia risked causing “catastrophic” energy price rises.

Putin’s top diplomat Sergei Lavrov clashed with his counterparts at a G20 meeting in Indonesia where they urged Russia to allow Ukraine to ship blockaded grain out to an increasingly hungry world.

Meanwhile, Russia’s envoy to London offered little prospect of a pull-back from parts of Ukraine under Russian control.

Ambassador Andrei Kelin told Reuters that Russian troops would capture the rest of Donbas in eastern Ukraine and were unlikely to withdraw from land across the southern coast.

Ukraine would eventually have to strike a peace deal or “continue slipping down this hill” to ruin, he said.

On the Donbas frontlines, Ukrainian officials reported Russian shelling of towns and villages ahead of an anticipated push for more territory.

A Ukrainian infantry unit on the road to the town of Siversk, whose members spoke to Reuters, had set up positions on the edge of a deep earth bunker covered with logs and sandbags and defended by machine guns.

“Right now it’s more of our artillery than theirs, so it’s good,” said the unit deputy commander. 

“But there might be incoming soon.”

On Thursday, Putin had indicated that current prospects of finding a solution to the conflict were dim, saying Russia’s campaign in Ukraine had barely started.

Ambassador Kelin’s remarks gave an insight into Russia’s potential endgame – a forced partition that would leave its former Soviet neighbour shorn of more than a fifth of its post-Soviet territory.

“Of course it is difficult to predict the withdrawal of our forces from the southern part of Ukraine because we have already experience that after withdrawal, provocations start,” Kelin said.

An escalation of the war was possible, he added.

Ukrainian officials, echoing comments by the deputy commander of the infantry unit outside Siversk, said they needed more high-grade foreign weapons to shore up the their defences.

Oleksiy Danilov, secretary of the National Security and Defence Council, said Ukraine still did not have enough weapons and soldiers needed time to adapt to using them.

US President Joe Biden signed a new weapons package worth up to $US400 million ($A583 million) for Ukraine on Friday, including four additional high mobility artillery rocket systems (HIMARS) and more ammunition.

The United States started providing the key precision rocket weapon system to Ukraine last month after receiving assurances from Kyiv that they would not be used to hit targets inside Russian territory. 

Ukraine has attributed battlefield successes to the arrival of the HIMARS.

“When they came in, the Russian war machine could instantly feel its effect,” Danilov told Reuters. 

But more military aid was vital, he added.

President Volodymyr Zelenskiy’s chief of staff also urged Ukraine’s allies to send more heavy weapons to counter what he called Russia’s “scorched earth tactics”.

“With a sufficient number of howitzers, SPG and HIMARS, our soldiers are able to stop and drive the invaders from our land,” Andriy Yermak wrote on Twitter.

Ukraine has not used HIMARS to strike Russian targets outside of Ukraine territory, a senior US defence official said on Friday, disputing Russian accusations.