Washington DC - A bipartisan group of senators has proposed legislation that would slap sanctions on countries friendly to Russia if Moscow disrupts ongoing negotiations to end the Ukraine war.

US President Donald Trump is trying to broker a ceasefire between Ukraine and its Russian invader, but his administration has failed to reach a breakthrough despite negotiations with both sides.

Ukraine this month agreed to a planned 30-day ceasefire, but Moscow has rejected the US-backed proposal and upped its rhetoric on wanting to install new leadership in Kyiv.

On Tuesday, 50 Republican and Democratic senators introduced a proposal to impose a 500% tariff on imports from countries that buy fuel and uranium from Russia, if Moscow "refuses to engage in good faith negotiations for a lasting peace with Ukraine."

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The sanctions would also be applied if Russia "initiates another effort, including military invasion, that undermines the sovereignty of Ukraine after peace is negotiated," the senators, led by Republican Lindsey Graham and Democrat Richard Blumenthal, said in a statement.

Trump has criticized both Russian President Vladimir Putin and Ukraine's Volodymyr Zelensky over the deadlocked peace talks.

Warming ties between Washington and Moscow since Trump's return to office and his threats to stop supporting Kyiv have bolstered Russia on the battlefield as it pursues its invasion.

Meanwhile, Ukraine has accused Russia of dragging out talks with no intention of halting its offensive, with fresh attacks on the northeastern border city of Kharkiv.