US President Joe Biden doubted on Wednesday that Russian President Vladimir Putin would use a tactical nuclear weapon, as Kiev pleaded for a rapid increase in Western military aid.

In an interview for CNN, Biden considered Putin a "rational actor who has miscalculated significantly."

Responding to a question about how realistic he believed it would be for the Russian President to use a tactical nuclear weapon, Biden said, "Well, I don’t think he will."

But the US President considered that the war in Ukraine may lead to miscalculations that could potentially end in the biblical event known as "Armageddon".

"Mistakes get made, and the miscalculation could occur, no one can be sure what would happen, and could end in Armageddon," he pointed out.

In the same context, Reuters cited a European diplomat as saying that NATO was considering convening a virtual summit to consider its response to Russia's nuclear threats, the accession of four regions, and the mobilization of troops.

On Tuesday, NATO Secretary-General Jens Stoltenberg told reporters in Brussels that the military alliance has not noticed any change in Moscow's nuclear posture following the threats.

Zelensky appeals for more air defence capabilities

On his part, Ukrainian President Volodymyr Zelensky indicated that he expects Wednesday a positive response from Western allies in Brussels to Kiev's requests for a rapid increase in military aid.

The Ukrainian President appealed Tuesday to the leaders of the Group of Seven nations for more air defense capabilities, and the G7 pledged to support Kiev for "as long as it takes."

In a statement, the G7 - which groups the United States, Germany, France, Japan, Britain, Italy, and Canada - vowed continued "financial, humanitarian, military, diplomatic and legal support ... for as long as it takes" to Ukraine.

Reuters reported that "a US-led coalition of some 50 countries known as the Ukraine Defence Contact Group will meet in Brussels on Wednesday on the sidelines of a NATO defence ministers meeting."

"I am anticipating from our partners progress on matters of anti-aircraft and anti-missile defences and agreements on new supplies of different weapons and ammunition vital for us," Zelensky said.

Four additional HIMARS from the US to Ukraine

Ukrainian Defence Minister Oleksii Reznikov announced on Twitter the arrival of four additional High Mobility Artillery Rocket Systems (HIMARS) from the United States.

Washington also announced that it was speeding up the shipment of NASAMS air defenses to Ukraine. It is noteworthy that the US has already provided Ukraine with more than $16.8 billion worth of military aid during the war.

A German Defense Ministry source confirmed that Ukraine on Tuesday also received the first of four IRIS-T air defense systems Germany promised to supply. 

It is noteworthy that Zelensky on Tuesday ruled out peace talks with Putin. This comes after Russian Foreign Minister Sergey Lavrov told state television that Russia was open to talks with the United States or with Turkey on ways to end the war in Ukraine, but Washington claimed that such offers were "posturing".