Germany, France and Britain have condemned what they said was Iran's export of ballistic missiles to Russia for use in the Ukraine war and declared new sanctions targeting air transport.
"We will be taking immediate steps to cancel bilateral air services agreements with Iran," they said in a statement, adding that they would also "work towards imposing sanctions on Iran Air".
US Secretary of State Antony Blinken has said that Iran has sent short-range missiles to Russia that are expected to be used against Ukraine within weeks, as he announced a joint solidarity trip to Kyiv with his British counterpart.
Mr Blinken, on a visit to London, said the United States and its allies would impose new sanctions on Iran for defying warnings on sending the missiles, including on state carrier Iran Air.
He said that dozens of Russian military personnel have trained in Iran using the Fath-360 missile, which has a range of 120km.
"We've warned Iran privately that taking this step would constitute a dramatic escalation," Mr Blinken told a news conference in London.
"Russia has now received shipments of these ballistic missiles and will likely use them within weeks in Ukraine against Ukrainians.
"Iran's new president and foreign minister have repeatedly said that they want to restore engagement with Europe. They want to receive sanctions relief. Destabilising actions like these will achieve exactly the opposite," he said.
Mr Blinken and Mr Lammy said they would travel together "this week" to Ukraine, in the first such joint trip in recent years.
"We are the closest of allies, so I'm delighted that we will travel together, demonstrating our commitment to Ukraine," Mr Lammy said.
Mr Blinken's trip comes ahead of a White House visit on Friday by UK Prime Minister Keir Starmer, his second trip to Washington since becoming prime minister.
While the US-UK "special relationship" has persevered across partisan lines, Labour has traditionally been closer in its core principles to President Joe Biden's Democratic Party.
But for Mr Starmer, a full alignment with the Democrats' agenda could carry risks just two months before US elections in which Mr Biden's political heir Kamala Harris is running neck and neck with Republican Donald Trump.
Mr Starmer has made clear that he will maintain the previous Conservative government's stance of staunchly backing Ukraine against Russia's invasion, with Britain frequently nudging the United States to ease restrictions on military support.
Last week, London said it was sending 650 new specialist missile systems to boost Ukrainian air defences, after President Volodymyr Zelensky criticised the pace of weapons deliveries.
Mr Lammy said, after Mr Blinken arrived last night, that London and Washington were "committed to supercharging our alliance", as "in a more volatile and insecure world, it is even more important that we are highly aligned nations".
Mr Starmer, however, has taken a harder line than the Conservatives on Israel, which relies on the United States as its foremost backer in the Gaza war.
His government last week announced restrictions on some weapons to Israel, voicing concern that they could be used to violate international humanitarian law.
The United States declined to criticise Britain's decision, saying that its ally had its own processes to make assessments, even after the US State Department's own review concluded there were no grounds to restrict weapons.
