The UK and US’s top diplomats will visit Ukraine in a show of solidarity later this week after secretary of state Antony Blinken formally accused Iran of supplying ballistic missiles to Moscow.

Foreign Secretary David Lammy hailed the trip as “the first of its kind in a decade”, at a press conference in London alongside his American counterpart on Tuesday.

Speaking to media in the Foreign Office’s Locarno Suite, Mr Lammy said the visit would be critical in order to allow both men to listen to Ukrainian concerns and report back to their leaders.

“It’s hugely important that we’re travelling together to hear from our Ukrainian counterparts and President Zelensky, their assessment of the situation on the ground and their needs on the ground,” Mr Lammy told journalists.

Meanwhile, Mr Blinken announced that Washington will deepen sanctions on Iran as he publicly accused Tehran of providing ballistic missiles to Russia.

“The United States will be announcing further sanctions on Iran later today, including additional measures on Iran Air,” Mr Blinken said.

“We expect allies and partners will be announcing their own new measures on Iran as well.”

The US has warned Iran that sending the weaponry to Russia to use in Ukraine would “constitute a dramatic escalation”.

“Russia has now received shipments of these ballistic missiles and will likely use them within weeks in Ukraine against Ukrainians,” he said.

The Iranian ballistic missile system, which has a maximum range of 75 miles, allows Russia to target its arsenal further from the front line, he added.

Iran, as it has with previous US intelligence findings, has denied providing Russia with weapons for its war in Ukraine.

Mr Lammy said the joint trip to Kyiv was the “first of its kind for a decade”, with both the Foreign Secretary and US secretary of state visiting their counterparts in Kyiv.

But he would not be drawn on whether Ukraine would be given permission to use UK-supplied Storm Shadow missiles on Russian soil, which Mr Zelensky has long called for.

“It would, however, be quite wrong to comment on the detail of operational issues in a forum such as this, because the only person who could benefit is (Russian president Vladimir) Putin, and we will do nothing to give him any advantage in his illegal invasion,” Mr Lammy said.

Mr Blinken’s UK trip comes ahead of Prime Minister Sir Keir Starmer’s second visit to the White House on Friday as part of efforts to strengthen diplomacy through what they describe as a new “UK-US bilateral strategic dialogue”.

The secretary of state said the relationship between the two allies went beyond special, as it has been described in the past, adding: “I like to call it essential.

“Essential to our nations, our people, and people well beyond our shores.”