The UK foreign secretary, David Lammy, has indicated that delicate negotiations with the White House to allow Ukraine to use Storm Shadow missiles inside Russia are ongoing, arguing it was a time for “nerve and guts”.

The apparent encouragement to Joe Biden comes just over a week after Lammy and Keir Starmer visited the US president in the White House but failed to resolve the sticking point between two countries.

Speaking at a fringe event at the Labour party conference in Liverpool, Lammy said the hardship and challenges of the war in Ukraine would get “deeper and harsher”, particularly heading into “the back end of 2025 into 2026” and beyond.

“So this is a critical time for nerve and guts and patience and for fortitude on behalf of allies who stand with Ukraine,” he said in comments that appeared directed at a hesitant White House, concerned about the risks of allowing Storm Shadow missiles to be used to attack Russia.