U.S. Secretary of State Antony Blinken is set to arrive in Ukraine on Wednesday, where he will meet with senior government officials at what he said was a critical moment for supporting the country in its fight against Russia's invasion.

The top U.S. diplomat, who is traveling to Ukraine alongside Britain's Foreign Secretary David Lammy, said he will use his visit to hear directly from Ukrainian President Volodymyr Zelenskiy and others what Kyiv's current goals in the war are and what Washington can do to help it achieve them.

"I think it’s a critical moment for Ukraine in the midst of what is an intense fall fighting season with Russia continuing to escalate its aggression," Blinken said in London at a news conference with Lammy.

Zelenskiy has been pleading for Western countries to supply longer-range missiles and to lift restrictions on using them to hit targets such as military airfields inside Russia.

On the battlefield more than 2-1/2 years since the invasion, Ukrainian forces are being stretched by a better armed and more numerous foe, as they try to fend off creeping Russian gains in the east where Moscow is focusing its attacks.

In a bid to seize back some of the initiative and divert Russian forces, Kyiv sent troops into Russia on an audacious large-scale cross-border incursion last month, but Moscow's troops have continued to inch forward in the east.

The visit also comes a day after Blinken in London said Russia has received ballistic missiles from Iran and will likely use them in Ukraine within weeks, warning that cooperation between Moscow and Tehran threatens wider European security.

The deepening military cooperation between Iran and Russia is a threat for all of Europe, Blinken said, and added that Washington had privately warned Iran that providing ballistic missiles to Russia would be "a dramatic escalation." The U.S. issued sanctions on Iran later on Tuesday over the transfer.

Blinken declined to say whether Washington will allow Ukraine to use long-range weapons deep inside Russia but said multiple factors went into the consideration of this decision rather than just looking at it as a weapons system.

"It’s not just the system itself that counts. You have to ask: Can the Ukrainians effectively use it, and sometimes that requires significant training, which we’ve done. Do they have the ability to maintain it?," Blinken said.

Zelenskiy has long pushed back against allies who have supplied long-range weapons but told Kyiv they cannot use them deep inside Russia for fear of instigating a direct conflict between the West and Russia's President Vladimir Putin.

Thousands of civilians have died in the war, which Russia started with a full-scale invasion on Ukraine in February 2022. Millions of Ukrainians have also been displaced, while their cities and villages have become piles of rubble.

Russia has escalated its drone and missile attacks on Ukraine in recent weeks, while Ukraine has also sent hundreds of long-range attack drones deep into Russian territory.

Later this month, Zelenskiy travels to the United States and will present a plan to President Joe Biden and his two potential successors in Novembers election that he hopes will bring the end of the war closer.