KYIV, Ukraine, Sept 11, 2024 (BSS/AFP) - The US and British top diplomats vowed Wednesday to work together for Ukraine's victory as they discussed 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 Labour government has vowed to keep up Britain's role as a key defender of Ukraine.

At three-way talks with their Ukrainian counterpart, Blinken said the visit sent a "strong message that we are committed to Ukraine's success, committed to Ukraine's victory".

Lammy also promised British support until the war of "Russian imperialism and aggression come to an end" and called attacks that have killed Ukrainians "horrific, barbaric, unbelievable".

"The only person that gains from any sense that we are not together" is Russian President Vladimir Putin, Lammy said. 

Paying his fifth visit to Kyiv since Russia's invasion, Blinken dropped by a celebrated restaurant to sample borscht, a bright red beetroot soup and Ukrainian national dish.

The trip comes at a fraught time for Ukraine, with Russia advancing on the key logistics hub of Pokrovsk in the eastern Donetsk region and 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.

US President Joe Biden, asked in Washington 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.

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

- Fears from Iran missiles -

Asked how Moscow would respond to such a development, Kremlin spokesman Dmitry Peskov told reporters 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.

Ukraine enjoyed a fresh boost late Tuesday when the International Monetary Fund said it reached a staff-level agreement that could open the door to $1.1 billion for the country, which is weathering Russian attacks on infrastructure as winter sets in.

But on the military front, the United States said it believes that Russia could start firing short-range Iranian-made missiles into Ukraine within weeks.

Cash-strapped Iran went ahead with the sale of the missiles despite repeated warnings from Western powers, which on Tuesday announced new sanctions on the cleric-run state.

The Iranian shipments have raised fears that Moscow would be freed up to use its long-range missiles against comparatively unscathed areas in western Ukraine.

The United States earlier this year gave its blessing for Ukraine to use Western weapons to hit Russian forces when in direct conflict across the border.

British media reports said Biden, who meets Prime Minister Keir Starmer on Friday, was set to end objections to letting Ukraine fire long-range Storm Shadow missiles into Russia. 

Britain has repeatedly pushed the United States, by far Ukraine's biggest military supplier, to be more forward on weapons.

One key ask by Ukraine is to loosen restrictions on US-supplied Army Tactical Missile Systems, or ATACMS, which can hit targets up to 300 kilometres (190 miles) away.

In a joint letter to Biden, leading members of Congress from the rival Republican Party asked him to act on ATACMS immediately.

"As long as it is conducting its brutal, full-scale war of aggression, Russia must not be given a sanctuary from which it can execute its war crimes against Ukraine with impunity," said the letter signed by Representative Mike McCaul, chairman of the House Foreign Affairs Committee.

Republicans, however, are deeply divided over Ukraine, and a victory in November by Republican presidential candidate Donald Trump over Biden's political heir Kamala Harris could dramatically shift US policy.

Trump aides have suggested that if he wins, he would leverage aid to force Kyiv into territorial concessions to Russia to end the war.