Ukraine's Dmytro Kuleba said his Russian counterpart Sergei Lavrov had indicated he did not have the authority to negotiate even a 24-hour ceasefire or a humanitarian corridor in Mariupol, the besieged southern Ukrainian city under heavy fire from Russian artillery.

Mr Lavrov left the door open for further talks and an eventual meeting between presidents Vladimir Putin and Volodymyr Zelenskyy.

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However, some military experts - and Prime Minister Boris Johnson - have shared scepticism about the Russian President backing down any time soon, while weighing up the options over UK and NATO eventually getting involved, as it would mean going into direct conflict with Russia.

William Alberque, from the International Institute of Strategic Studies, told Sky News: "If they do a ceasefire, all Russia is going to do is buy time to resupply, to remove his exhausted troops, to replace them with another group of troops in three or six months time and start all over again.

"This is about ultimately toppling the government in Ukraine and replacing it. But I do think the Ukrainian fighters can hold on, the bravery they've shown is staggering and if they just delay the Russians long enough until they are exhausted, we just get into a ceasefire.

"But then I think this will just start again. So Western support for Ukraine is critical for the survival of this country."

He added the situation showed how "Mr Putin operates", as he likes to have as many options as possible.

He said: "So if My Zelenskyy wants to give up, that's fine, he is still going to go for his goal which is to overthrow the government.

"This is all part for show and part to see if Ukraine will surrender.

"Ukraine knows there's nothing Russia will accept, except for total surrender."

Mr Alberque also said the West may "have to act" if chemical weapons are used.

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He said: "I really don't think he understands the resilience of the Ukrainian people - that they will all fight. He has been so steeped in his own propaganda.

"I think Ukraine is going to fight, unfortunately to the last drop of Ukrainian blood.

"I think in order to demoralise the population, I think as a false flag operation he will use chemical weapons.

"But I hope not because I do think we're going to have to act."

Anders Fogh Rasmussen, former head of NATO, called for "an outright economic war against Russia" in order to stop Mr Putin causing more pain other countries too.

He told Sky News: "The Russians are not advancing as planned so Mr Putin will get more desperate and that will lead to more devastating warfare.

More: Boris Johnson fears Vladimir Putin may use chemical weapons as it is 'straight out of Russia's playbook'

"We should step up our delivery of weapons to Ukraine - they have demonstrated great bravery and we should help them all we can… with drones, with anti-tank equipment, better air defence systems, etc.

"It is indeed heartbreaking to watch the images of Russian bombings of civilians. At this stage I would not exclude anything.

"We are confronted with a political gangster in the Kremlin - and in defence against gangsters, you should not exclude any action.

When asked about a no-fly zone or some form of military intervention, he added

He said: "First of all I would declare economic war against Russia. I think that is what is needed now to stop the oil and revenue to completely cut Russia off the international economic system.

"But concerning the no-fly zone - I think it should still be on the table.

"However, Russia would consider it a declaration of war and Mr Putin only respects one thing - force and determination and unity

"We shouldn't exclude anything. He should be kept in doubt about our intentions

"I would suggest the EU immediately stop all purchase of oil and gas from Russia.

"I think we should follow the developments very closely and if Russia continues to target civilians as we have seen, I would consider that crossing a red line."

When asked about whether this would lead to military action, he said: "Of course we have to think this through very thoroughly because a no-fly zone might make more harm than good by expanding beyond Ukraine's borders.

"My preferred option would be to declare an outright economic war against Russia.

"We can handle it in economically - it will be tough - but that price would be very low and a tiny price compared to the loss of freedom if we do not act now."

He shared concern that Mr Putin "won't stop in Ukraine".

He said: "The next step would be Moldova and Georgia or even the Baltic states. We may have a choice of confronting Mr Putin now or later.

"The suffering and the losses might be less by confronting it at an early stage, rather than too late.

"I think if he gets good advice he would realise that Russia has no chance whatsoever against NATO.

"I do not want a military confrontation with Russia, I think a declaration of economic war would be more efficient.

"Mr Putin is definitely behaving irrationally because rational thinking wouldn't have led him to attack Ukraine and he has through an incredible change of mind."

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Meanwhile, Prime Minister Boris Johnson said that while Mr Putin had abandoned "all norms of civilised behaviour", a no-fly zone would bring the UK and NATO into direct conflict with Russia, although this was something he was determined to avoid.

Speaking on Sky News Beth Rigby Interviews programme, he acknowledged that some of his conversations with Mr Zelenskyy had been "deeply upsetting" as the Ukrainian leader appealed for more help.

He said: "What's happened in Mariupol in that maternity hospital really shows that Mr Putin is prepared just to reject, to abandon, all norms of civilised behaviour.

"The difficulty is that there is a line beyond which, quite frankly, the UK and NATO would be deemed to be in conflict - direct conflict - with Russia.

"It's agonising. It's absolutely agonising. And I've had this conversation at least a couple of times now with Volodymyr, but I think the difficulty is that it will require me to order RAF jets, UK pilots into the air with a mission to shoot down Russian fast jets.

"I think we've got to be realistic... there's a line that is very difficult to cross."