President Volodymyr Zelensky says Ukraine will make no concessions on its “territorial integrity” as officials prepare for the first face-to-face peace talks with Russia in more than a fortnight.

The meeting is due to be held in Istanbul on Tuesday (Australian time) as Russia’s invasion has become bogged down and the battlefield momentum has shifted in Ukraine’s favour.

It comes as a media investigation revealed billionaire Russian oligarch Roman Abramovich and two Ukrainian delegates suffered suspected poisoning at an earlier round of peace talks in Kyiv (more details below).

Ukrainian officials have played down the chances of a major breakthrough, but the fact that they are about to hold more discussions in person at all was a sign of shifts behind the scenes.

At the last acrimonious in-person meeting on March 10, Ukraine accused Russian Foreign Minister Sergei Lavrov of ignoring its pleas to discuss a ceasefire.

And Mr Lavrov said a halt to fighting was not even on the agenda.

Since then, both sides have repeatedly met via video link and have publicly discussed progress on a diplomatic formula under which Ukraine might accept some kind of formal neutral status.

But neither has budged over Russia’s territorial demands, including Crimea, which Moscow seized and annexed in 2014, and eastern territories known as the Donbas, which Moscow demands Kyiv cede to separatists.

In his latest comments overnight, President Zelensky made clear that “territorial integrity” remained Kyiv’s priority at the talks.

Last week, Ukrainian forces went on the offensive, pushing Russian troops back in areas around Kyiv, the northeast and the southwest.

Russia has meanwhile kept up pressure in the southeast near separatist areas, including its devastating siege of the port of Mariupol, razed to the ground with tens of thousands of civilians trapped inside for weeks.

As its offensive appears to be stalling elsewhere in Ukraine, most of Russia’s gains have been near Mariupol and heavy fighting is still underway there.

The mayor said Mariupol was on the brink of a humanitarian catastrophe with 160,000 people were still trapped inside, and he accused Russia of of trying to remove the city from the face of the earth.

Ukraine’s Deputy Prime Minister Iryna Vereshchuk said there were no plans to open corridors to evacuate civilians from besieged cities because of intelligence reports of possible Russian “provocations” along the routes.

Elsewhere, Russia’s armoured columns are bogged down, with trouble resupplying and making little or no progress, despite pounding residential areas.

Britain’s defence ministry also said there had been no major change in Russia’s positions in the past 24 hours.

Ukraine’s General Staff said Kyiv defence forces were holding back Russian troops trying to break through from the northeast and northwest and take over key roads and settlements.

Suspected poisoning of delegates

Reports have emerged that Russian billionaire Roman Abramovich and two Ukraine delegates who had attended peace talks on March 3 in Kyiv suffered symptoms of poisoning.

The investigative journalism group Bellingcat and the Wall Street Journal broke the story and said Mr Abramovich and the negotiators had symptoms “consistent with poisoning with chemical weapons”.

These included pain in the eyes, peeling skin and dizziness and for a time Mr Abramovich lost his eyesight.

The football club owner, who is said to be close to Vladimir Putin, was later treated in a Turkish hospital for what is believed to have been a mild dose of chemical poisoning.

The three apparent victims experienced symptoms on the evening of the talks, according to the investigative website Bellingcat.

They had reportedly eaten nothing more than chocolate and water.

Publicly, there has been no explanation for their unusual conditions and the US has suggested poisoning from the environment.

Mr Abramovich is the owner of the Chelsea Football Club but has stated he will be selling the club in the wake of Russia’s invasion and global sanctions on oligarchs.

The poisoning story has revealed his behind-the-scenes role in peace negotiations.

Row over Russian gas

Russia says it will not supply gas to Europe for free as it works out methods for accepting payments for its gas exports in roubles, but G7 countries have refused the demand.

At a meeting of European Union leaders on Friday, no common position emerged on Russia’s demand last week that “unfriendly” countries must pay in roubles, not euros, for its gas.

The Russian central bank, the government and Gazprom, which accounts for 40 per cent of European gas imports, should present their proposals for rouble gas payments to President Vladimir Putin by March 31.

“We are not going to supply gas for free, this is clear,” Kremlin spokesman Dmitry Peskov told a conference call.

“In our situation, this is hardly possible and appropriate to engage in charity (with European customers).”

Russia will take decisions in due course should European countries refuse to pay in the Russian currency, he added.

Meanwhile, energy ministers from the G7 industrialised countries rejected the rouble payment demands.

The EU aims to cut its dependency on Russian gas by two-thirds this year and end Russian fossil fuel imports by 2027.

-with AAP