It comes as US lawmakers have vowed to approve an extensive new weapons package for Kyiv. Russian forces have recalibrated their focus on the east and south after failing to capture the capital as they tried to overcome stiff Ukrainian resistance.
Moscow has managed to capture the city of Kherson in the south and is in control of the majority of the port city of Mariupol after a period of brutal fighting, despite some remaining resistance in a steel plant.
The United Nations is undertaking efforts to evacuate civilians from the embattled city by creating humanitarian corridors.
Ukraine and Russia have not held face-to-face peace talks since March 29, although there have been talks by video link.
Relations between Moscow and Kyiv have worsened in the last month following allegations of Russian atrocities.
Kyiv has been particularly angry about the behaviour of Russian troops in the suburbs of the capital before Moscow’s forces withdrew, with the deaths of civilians in Bucha being the most egregious example.
Ukrainian President Volodymyr Zelensky expressed pessimism about future talks when talking to Polish journalists according to Interfax.
He said: “The risks that the talks will end are high because of what they (the Russians) have left behind them, the impression that they have a playbook on murdering people.”
Moscow launched its invasion in February on the pretext of preventing Ukraine from joining NATO.
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Russian Foreign Minister Sergei Lavrov said that Kyiv would have been given security guarantees from the UN Security Council had it been “honest” in negotiations.
Mr Lavrov added in an interview with Russian news agencies that Moscow became “stuck” because of Kyiv’s “inconsistency” in comments attempting to justify the invasion.
He said: “We got stuck because of their inconsistency, because of their desire to play games every time and – as far as I can guess – because of instructions they get from Washington, London and other capitals not to speed up the negotiating process.”
Both the United States and Britain have supported Ukrainian participation in the talks but have argued that weapons still need to be sent to Kyiv.
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On Thursday US President Joe Biden asked congress to fund $33 billion (£26 million) in new aid, including over $20 billion (£16 billion) in weapons.
The funding has received bipartisan congressional support and House of Representatives Speaker Nancy Pelosi said she hoped the legislation to pass the funding would be on the statute books “as soon as possible”.
Russian President Vladimir Putin has labelled Moscow’s actions a “special military operation” to disarm Ukraine, defend Russian-speaking people from persecution and prevent the United States from using the country to threaten Russia.
Ukraine has dismissed Putin’s arguments about the persecution of Russian speakers in the east.
Kyiv argues that Moscow’s actions in the Donetsk and Luhansk regions amount to an imperial land grab.