The US secretary of state, Antony Blinken, gave his strongest hint yet that the White House is about to lift its restrictions on Ukraine using long-range weapons supplied by the west on key military targets inside Russia, with a decision understood to have already been made in private.

Speaking in Kyiv alongside the UK foreign secretary, David Lammy, Blinken said the US had “from day one” been willing to adapt its policy as the situation on the battlefield in Ukraine changed. “We will continue to do this,” he emphasised.

Blinken said he and Lammy would report back to their “bosses” – Joe Biden and Keir Starmer – after their talks on Wednesday with the Ukrainian president, Volodymyr Zelenskiy.

The foreign secretary suggested Iran’s dispatch of ballistic missiles to Moscow – revealed this week – had changed strategic thinking in London and Washington. It was a “significant and dangerous escalation”, he said.

He added: “The escalator here is Putin. Putin has escalated with the shipment of missiles from Iran. We see a new axis of Russia, Iran and North Korea.” Lammy urged China “not to throw in its lot” with what he called “a group of renegades”.

British government sources indicated that a decision had already been made to allow Ukraine to use Storm Shadow cruise missiles on targets inside Russia, although it is not expected to be publicly announced on Friday when Starmer meets Biden in Washington DC.

The two leaders are planning to discuss the war in Ukraine, and how it could be ended, as part of a wide-ranging foreign policy discussion, though they will avoid an intense focus on any individual weapons system, as the aim of the conversation is strategic.