Allies must show “guts” and “nerve” in their support for Ukraine, David Lammy has said after the country’s ambassador to Britain warned permission to use western missiles within Russia was “critical” for Kyiv’s success.

The Foreign Secretary said international partners were engaged in a “very real-time discussion” about how to help, hinting at further developments with crunch talks on the conflict expected at a UN summit next week.

Speaking at a Labour Party fringe event alongside Ukraine’s ambassador to the UK, Valerii Zaluzhnyi, he warned it was a “critical time” for allies to show “fortitude” and “patience” in bolstering Ukraine’s defence.

There is a very real-time discussion across allies about how we can support Ukraine as we head into the winter

“This is a critical time for nerve and for guts and for patience and for fortitude on behalf of allies who stand with Ukraine,” Mr Lammy said.

“I am not going to, as Foreign Secretary, of course, comment on operational detail, because that can only aid Putin.

“But there is a very real-time discussion across allies about how we can support Ukraine as we head into the winter.”

The UK and US have stopped short of explicitly giving Kyiv permission to fire western long-range missiles – such as Britain’s Storm Shadow weapons – at targets within Russia despite repeated pleas from the country’s government.

Western concerns include the risk of escalation, with Russian President Vladimir Putin warning such a move would mean that Nato countries were at war with Moscow in the wake of speculation that a change of position could be imminent.

But speaking on Sunday, Mr Lammy said: “I think we have to recognise Putin’s bullying bluster and rhetoric for what it is: a sort of deliberate psychological warfare that is usually indicative of his own fears about losing this ridiculous and horrendous bloody escapade of his. He tends to throw dust up in the eyes.”

He added: “Now, look, there is a challenge. The challenge is that obviously in a democracy with a free press that, of course, underpins our democracy, and is important in our democracy, there is a sort of public debate and discussion, and sometimes that discussion becomes quite detailed in terms of tactics and how you prosecute a war in a way that the tactics of Russia, Iran and North Korea are not public.

“That relies on the intelligence capability of us and our Five Eyes partners particularly, in sometimes revealing what we know what is happening. So, there is this imbalance between democracies where obviously there is a healthy discussion, and autocracies… and we’ve just got to get that balance right.

“I have to err on the side of not going into too much detail about how we are assisting Ukraine and are going to continue assisting Ukraine.”

Earlier in the event, Mr Zaluzhnyi insisted that lifting restrictions on the weapons would be crucial to Kyiv’s defence.

He said: “Modern weapons with long-range air and ground abilities are critically important.

“Lifting restrictions on use of weapons against military targets in Russia is critical. This would help protect civilians from Russian missiles and … bombs.

I believe in the resilience of the Ukrainian people, but without additional help the price will be very, very high

“This winter in Ukraine will be the most difficult. As we know, I believe in the resilience of the Ukrainian people, but without additional help the price will be very, very high.”

Sir Keir Starmer travelled to Washington earlier this month to discuss the issue with US President Joe Biden but no decision was made, with both leaders continuing to resist green-lighting use of the missiles.

It is likely to come up again when Sir Keir Starmer, Mr Biden and Mr Zelensky all attend the UN General Assembly in New York next week.

The Ukrainian president has heightened his appeal for permission in recent weeks, insisting decisiveness “most effectively protects against terror” and urging leaders to overcome the “fear” of making the choice.