United States President Joe Biden told reporters in Washington Tuesday that his administration is working out whether to let Ukraine fire US longer-range missiles into Russia.

"We're working that out right now," Biden said, when asked by a reporter.

American and British top diplomats are visiting Kyiv, Ukraine on Wednesday to discuss further easing rules on firing Western weapons into Russia, whose alleged acquisition of Iranian missiles has raised new fears.

In a rare joint trip, US Secretary of State Antony Blinken took the nine-hour train from Poland to Kyiv alongside Foreign Secretary David Lammy, whose two-month-old Labor government has vowed to keep up Britain's role as a key defender of Ukraine.

The visit comes as President Volodymyr Zelensky ramps up his requests to the West to provide weapons with more firepower and fewer restrictions.

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

Blinken, speaking Tuesday in London alongside Lammy, said the US was committed to providing Ukraine "what they need when they need it to be most effective in dealing with the Russian aggression."

But Blinken, who is paying his fifth trip to Kyiv since the invasion, said it was also important to see if Ukrainian forces could maintain and operate particular weaponry.

Pressed later in an interview with Sky News on whether the US would green-light long-range weapons, Blinken said, "We never rule out, but when we rule in, we want to make sure it's done in such a way that it can advance what the Ukrainians are trying to achieve."