Mr Zelensky will address both the Nato and EU summits. Nato leaders, including US President Joe Biden, who is attending all three summits, have ruled out the no-fly zone because it would risk triggering a war with Russia. 

Mr Biden will call on allies to step up support to Kyiv and boost sanctions against Moscow. 

The Ukrainian president urged Western allies to stay united in the face of Russian pressure. He accused “some partners” of undermining the united front on Russia’s behalf. 

“We will see who is a friend, who is a partner and who has sold out and betrayed us,” he said in the emotional speech. ‘Together we should not allow Russia to break anyone in Nato, the EU or G7, to break them and drag them to the side of war.”

Mr Zelensky did not clarify who he suspected of going soft on Vladimir Putin. However Germany and Hungary, which are both dependent on Russian energy, have opposed new EU sanctions on Russian oil and gas. 

‘The most serious crisis in a generation’

German Chancellor Olaf Scholz, who reversed decades of policy to ramp up defence spending to more than the Nato target of 2 per cent GDP, has warned such sanctions could trigger a recession in Germany. 

Nato Secretary-General Jens Stoltenberg said allies were meeting to face “the most serious security crisis in a generation.”

He predicted that allies would agree to send more equipment to Ukraine and bolster their defences on the Eastern flank with a further four battlegroups

“The leaders will focus on our support to Ukraine. Allies have provided support to Ukraine for several years and we have stepped up with more military support, financial support, humanitarian support to help Ukraine uphold its fundamental right for self-defence,” he said. 

The equipment will include gas masks and protective suits, respirators, hand held detectors and mobile decontamination units to help protect Ukrainian troops against feared chemical and nuclear weapons attacks. 

Mr Stoltenberg warned that any chemical attack by Russia would “fundamentally change the nature of the conflict.”

Such an attack would risk the fallout spreading into Nato territory, he said, but refused to say if it would provoke a military response from the Alliance. 

He said, “It would be a blatant violation of international law, and it will have wide-spread and severe consequences.”

“There is also a risk that we can see a spread of chemical agents into Nato territory,” he added, “I will not speculate beyond the fact that Nato is always ready to defend, to protect and to react to any type of attack on a Nato allied country.”