The United States and Britain on Wednesday committed to nearly US$1.5 billion in support for Ukraine and promised to consider quickly requests to ease restrictions on weapons to strike deeper into Russia.

US Secretary of State Antony Blinken and UK Foreign Secretary David Lammy paid the rare joint visit to Kyiv in a show of solidarity as concerns mount.

Russia's alleged acquisition of new short-range Iranian missiles threatens to ramp up Moscow's firepower and the US election in less than two months could dramatically shift the stance of Ukraine's top backer.

Blinken, who took a nine-hour train ride to Kyiv alongside Lammy, said the United States would offer US$717 million in new economic help to Ukraine.

About half of it will consist of US support to strengthen Ukraine's electricity infrastructure, which has been pounded by Russia just as winter sets in.

Blinken accused Russian President Vladimir Putin of dusting off "his winter playbook" to "weaponise the cold against the Ukrainian people."

"Our support will not wane, our unity will not break," Blinken told a joint news conference in Kyiv.

"Putin will not outlast the coalition of countries committed to Ukraine's success, and he is certainly not going to outlast the Ukrainian people."

Lammy recommitted his two-month-old Labour government to providing 600 billion pounds (US$782 million) in economic assistance to Ukraine.

He said that Britain, which has consistently pushed to ease restrictions on Ukraine's use of weapons, would provide hundreds of new air defence missiles to Ukraine this year.

Russia has been advancing on the key logistics hub of Pokrovsk in the eastern Donetsk region, a month after Kyiv launched a shock counter-offensive into Russia's Kursk region.

President Volodymyr Zelensky has ramped up his requests over recent months to the West to provide weapons with more firepower and fewer restrictions.

"It is important to lift any restrictions on the use of US and British weapons against legitimate military targets in Russia," Ukrainian foreign minister Andriy Sybiga told the joint conference with his counterparts, who earlier met Zelensky.

Blinken said that the United States would look at the requests "with urgency" and that they would be discussed on Friday in Washington when President Joe Biden meets UK Prime Minister Keir Starmer.

Biden, asked in Washington on Tuesday whether he would let Ukraine use longer-range weapons for strikes on Russian targets, said: "We're working that out right now."

Biden, while strongly supportive of Ukraine, has previously made clear he wants to avoid devolving into direct conflict between the United States and Russia, the world's two leading nuclear powers.

Asked how Moscow would respond to expanded missile ranges, Kremlin spokesman Dmitry Peskov told reporters on Wednesday the response "will be appropriate," without providing specific details.

He said the authorisation of Ukrainian strikes on Russian territory would serve as "further proof" of why Moscow launched its offensive, which he said was itself an "answer" to the West's support for Ukraine. (AFP)