President Joe Biden has authorised Ukrainian forces to use American missiles to strike targets deep inside Russia, US officials claimed.

Washington gave Kyiv the green light to use the Army Tactical Missile Systems, (ATACMS), after North Korean troops joined Moscow’s soldiers in the conflict.

Fears are intensifying Vladimir Putin’s forces could launch a major assault, with an estimated 50,000 soldiers, on Ukrainian positions in Kursk.

The Kremlin is believed to want to retake all of the Russian territory seized by Kyiv in August.

The UK will be hoping that President Biden’s decision will allow Ukrainian troops to use British-made Storm Shadow missiles.

And Ukrainian forces could now use the ATACMS to strike Russian and North Korean troops, key pieces of military equipment, ammunition depots, supply lines deep inside Russia and logistics hubs.

Washington had been concerned that allowing President Zelensky’s forces to use the weapons could lead to another escalation of the war.

US officials were deeply divided, with some believing that Western hesitancy had allowed Russia to gain an advantage on the battlefield.

But others seized on a US intelligence assessment that warned Putin could order his military – or spy agencies – to retaliate against the US and its European allies.

The sight of thousands of North Koreans arriving and donning Russian uniforms was said to have persuaded Mr Putin to switch positions.

Prime Minister Sir Keir Starmer has ruled out talks with Russian tyrant Vladimir Putin as he urged European allies to “double down” on support for Ukraine.

Sir Keir will tell World leaders in Brazil that the 1000^th day of the war should prompt them to go “further and faster in their support of” Kyiv.

He is expected to urge US President Joe Biden to sign off on Ukrainian troops firing British-made Storm Shadow missiles deep into Ukraine.

The West’s support for Ukraine has been drawn to sharp focus again after German Chancellor Olaf Scholz spoke with the Russian dictator last week.

Kyiv has branded some of Mr Scholz’s comments akin to appeasement.

Sir Keir said: “It’s a matter for Chancellor Schloz who he speaks to. I have no plans to speak to Putin.

“We are coming up to the 1000th day of this conflict on Tuesday, that’s 1000 days of Russian aggression, a 1000 days of huge impact and sacrifice in relation to Ukrainian people and recently we’ve seen the addition of North Korean troops working with the Russians, which does have serious implications.

“I think on the one hand it shows the desperation of Russia but its got serious implications for European security and for Indo-Pacific security.

“That’s why I think we need to double down on shoring up our support for Ukraine and that’s top of my agenda for the G20.

“There’s got to be full support for as long as it takes and that certainly is top of my agenda, shoring up that further support for Ukraine.”

Sir Keir again dodged questions over British demands for Ukraine to be allowed to use Storm Shadow missiles.

But the Prime Minister said “I am going to make shoring up support for Ukraine top of my agenda.”

Kyiv is said to be concerned that Western support, without America, will ebb away.

Ukraine’s President Zelensky on Friday accused Germany of appeasement after Mr Scholz spoke to Russian tyrant Vladimir Putin for the first time since December 2022.

He said: “Olaf’s call has opened a Pandora’s box.

“Now there may be other conversations, other calls.

“This is exactly what Putin has long wanted. It is extremely important for him to weaken his isolation.

“I don’t think Putin wants peace at all. But that does not mean he doesn’t want to sit down with World leaders… and it benefits him to sit down, talk and not reach an agreement.”

Mr Zelensky has also said new President-elect Donald Trump will help end the war faster.

He added: “We must do everything we can to ensure this war ends next year.

“We have to end it by diplomatic means.”

Mr Scholz defended his phone call with Putin before leaving for the G20, insisting it was “important” to signal the West will continue to support Ukraine.

Britain has also faced questions over its support for Ukraine, with Kyiv said to have becoming increasingly frustrated over its slowing down of military support.

Labour is accused of not supplying any Storm Shadow to Kyiv, even to strike targets in Crimea or the Black Sea.