A top Russian official has said it could go to war with NATO if Ukraine strikes inside Russia with US weapons.

On Thursday, President Joe Biden partially lifted restrictions on how Ukraine can use military supplies sent by the US.

In response, senior Russian security official Dmitry Medvedev issued a series of warnings to the West.

“Russia regards all long-range weapons used by Ukraine as already being directly controlled by servicemen from NATO countries. This is no military assistance, this is participation in a war against us,” Medvedev said.

“And such actions could well become a casus belli [an act that provokes a war].”

He said it would be a “fatal mistake” on the part of the West to think that Russia was not ready to use tactical nuclear weapons against Ukraine – and spoke of the potential to strike unnamed hostile countries with strategic nuclear weapons.

“This is, alas, neither intimidation nor bluffing,” said Mr Medvedev.

Ukraine launches major attack against Russian base in Crimea

Ukraine war: Crowded DIY store in Kharkiv hit by Russian airstrikes, killing at least 14 people

Vladimir Putin ready to ‘freeze’ war in Ukraine with ceasefire recognising recent Russian gains, sources say

He said Moscow’s conflict with the West was developing according to the worst case scenario and that “nobody today can rule out the conflict’s transition to its final stage”.

Read more: Ukraine-Russia war latest updates

The partially-lifted restrictions from the US mean Ukraine can use American weapons to strike inside of Russia but only in defence of Kharkiv.

Please use Chrome browser for a more accessible video player

However, US officials stressed that Ukraine still cannot use American long-range missiles and other munitions to strike inside Russia offensively.

Ukrainian President Volodymyr Zelenskyy and others have been increasingly vocal that US restrictions meant Ukrainian forces could not defend themselves as Russia intensified attacks around the north-east Kharkiv region.

International correspondent

The frontlines on Ukraine’s north eastern border around Kharkiv are looking increasingly shaky.

The second city is now in striking distance of Russian artillery.

Washington’s change in policy is limited in scope but it means the defenders will now be able fire US munitions into Russia in this area only, to try and stop Russian forces advancing.

Again, the emphasis is being placed on defensive action rather than offensive operations – but that is a matter of interpretation and will look very different in Kyiv, Washington and, of course, Moscow.

Using American weapons for attacks inside Russia has always been a red line for the Americans.

Since the conflict started, the Kremlin has continuously played up with bellicose rhetoric that such moves could lead to a direct conflict with America – and the spectre of a nuclear war.

But the precarious situation in North East Ukraine and the subsequent appeals from the Zelensky administration appear to have changed the calculation of policy makers in Washington.

It marks a sharp departure from where the Americans were at the beginning of the war.

Then the supply of weapons was only meant to be defensive in nature and to be used inside Ukrainian territory.

Now you have a situation where American guns and rocket launchers can be used to target positions inside a nuclear-armed country and enemy.

Russia’s response has been predictable – it’s warning that the move is a serious escalation and will have serious consequences.

The move, though, is unlikely to change the course of the war.

For many months Ukraine has been striking deeper inside Russian territory and it has not changed Russia’s will to fight.

But it’s hoped the power of American weapons will help Ukraine stabilise an area that has been under constant attack for the last three weeks and looks increasingly weak.

But it also increases the possibly of a direct confrontation with Russia in the future.

A Putin-backed thinktank also suggested on Thursday that Russia should consider a “demonstrative” nuclear blast to scare Ukraine from using Western weapons inside its territory.

Read more from Sky News:
Ukraine launches major attack against Russian base in Crimea
Crowded DIY shop hit by Russian airstrikes, killing at least 14

Mr Medvedev, the senior Russian official responsible for Friday’s threats, frequently makes dire threats to both Ukraine and the West, often invoking Russia’s nuclear weapons.

Some commentators even say his wild statements could be designed to ensure he is not viewed as statesmanlike and therefore not considered a threat to Mr Putin’s authority.

Keep up with all the latest news from the UK and around the world by following Sky News

Be the first to get Breaking News

Install the Sky News app for free

Although many do not attach huge significance to his threats, they could give a flavour of what the Kremlin is thinking, according to diplomats.

Related posts:

Children as young as 11 ‘being beheaded’ amid Islamist insurgency in Mozambique Before and after images show devastation caused by Germany floods Putin set to decide whether to recognise separatist regions in Ukraine as independent North Korea confirms Kim Jong Un’s new missile launch and releases photos The high-profile Russians who have fled Putin’s regime, quit their jobs and refuse to support Ukraine war Heavy snowfall causes 13 dead and more than 10,000 homes without power in Japan Pele’s family spending Christmas in hospital after football legend’s cancer progressed ‘Fire tourists’ urged to keep away as blazes force hundreds from their homes in eastern Spain Four people ‘critical’ after explosion in Paris sparks fire and reduces buildings to rubble ‘We don’t know what they are’: Search for origin of ‘noises’ will continue in sub hunt, Coast Guard says World entering ‘more dangerous period’ as countries massively up military spending Haiti PM to resign as powerful gang leader calls on international community to give the country a ‘chance’