Russia's Foreign Minister says Moscow's overarching goal in Ukraine is to topple President Volodymyr Zelenskyy's government, expressing the Kremlin's war aims in some of the bluntest terms yet, as its forces pummel the country with artillery barrages and airstrikes.

The remark from Sergei Lavrov comes amid Ukraine's efforts to resume grain exports from its Black Sea ports, something that would help ease global food shortages, under a new deal tested by a Russian strike on Odesa over the weekend.

Speaking to envoys at an Arab League summit in Cairo, Mr Lavrov said Moscow is determined to help Ukrainians "liberate themselves from the burden of this absolutely unacceptable regime".

Mr Lavrov accused Kyiv and its Western allies of spouting propaganda intended to ensure that Ukraine "becomes the eternal enemy of Russia". 

"Russian and Ukrainian people would continue to live together," he said.

"We will certainly help Ukrainian people to get rid of the regime, which is absolutely anti-people and anti-historical."

Mr Lavrov's remarks contrasted with the Kremlin's line early in the war, when it repeatedly emphasised that Russia wasn't seeking to overthrow Mr Zelenskyy's government, even as Moscow's troops closed in on Kyiv.

Russia later retreated from around the capital and turned its attention to capturing Ukraine's industrial Donbas region in the east.

The fighting is now in its sixth month.

Mr Lavrov argued that Russia was ready to negotiate a deal to end hostilities in March when Kyiv changed tack and declared its intention to rout Russia on the battlefield.

He said the West has encouraged Ukraine to keep fighting.

It was not yet clear when grain shipments would resume following Russia and Ukraine's signing of agreements with the United Nations and Turkey on Friday.

Russia defends attack on Odesa port

The deals are aimed at clearing the way for the shipment of millions of tonnes of desperately needed Ukrainian grain, as well as the export of Russian grain and fertiliser.

While Russia faced accusations that the weekend attack on the port of Odesa amounted to reneging on the deal, Moscow insisted the strike would not affect grain shipments.

During a visit to Congo on Monday, Mr Lavrov repeated the Russian military claim that the strike targeted a Ukrainian navy boat and a depot with Harpoon anti-ship missiles supplied by the West.

He said the attack took part in the military section of the port at "a significant distance" from the grain terminal.

"We haven't created any obstacles to grain deliveries in accordance with the agreements signed in Istanbul," Mr Lavrov said.

Russia tightens squeeze on gas flow to Europe 

Russia's Gazprom said on Monday that it would further reduce natural gas flows through a major pipeline to Europe to 20 per cent of capacity, citing repairs of equipment.

The Russian state-owned company tweeted that it would reduce "the daily throughput" of the Nord Stream 1 pipeline to Germany to 33 million cubic meters as of Wednesday.

The head of Germany's network regulator confirmed the reduction.

Deliveries on Monday were at 40 per cent of full capacity when Nord Stream 1 reopened after 10 days of scheduled maintenance last week.

The German government said it rejected the notion that technical reasons would lead to further gas reductions.

Natural gas is used to keep industry humming, generate electricity and heat homes in the winter, and concerns are rising about a possible recession if Europe does not save enough gas and rationing is required to get through the cold months.

Russia has cut off or reduced natural gas to a dozen European Union countries.

Russia recently has accounted for about a third of Germany's gas supplies.

The government said last week that the reduction in gas flows confirmed that Germany can't rely on Russian deliveries, announcing that it would step up its gas storage requirements and take further measures to conserve supplies.

Continuing offensives in Donetsk 

Meanwhile, Ukraine's presidential office said Monday at least two civilians were killed and 10 wounded in Russian shelling over the preceding 24 hours.

In the eastern Donetsk region, the focus of the Russian offensive, Russian artillery struck the cities of Avdiivka, Kramatorsk and Kostiantynivka.

An airstrike on Bakhmut damaged at least five houses.

"The Russians are using the scorched-earth tactics across the entire Donbas," Donetsk Governor Pavlo Kyrylenko said in televised remarks.

"They fire from the ground and from the air to wipe off entire cities," he said.

The Russians also struck the Kharkiv region.

In the city of Chuhuiv, a Russian strike wrecked a local club, and rescue workers removed several people from under the debris.

Kharkiv Governor Oleh Sinyehubov denounced the attack as "senseless barbarity". 

AP