The threat of Russian president Vladimir Putin using tactical nuclear weapons is “real”, his US counterpart Joe Biden said, days after denouncing Russia’s deployment of such weapons in Belarus.

“When I was out here about two years ago saying I worried about the Colorado river drying up, everybody looked at me like I was crazy,” Biden told a group of donors in California on Monday.

“They looked at me like when I said I worry about Putin using tactical nuclear weapons. It’s real,” Biden said.

Mr Biden had termed Mr Putin’s announcement that Russia had deployed its first tactical nuclear weapons to Belarus “absolutely irresponsible” last week.

This comes as Belarusian president Alexander Lukashenko said his country has started taking delivery of Russian tactical nuclear weapons, some of which he said were three times more powerful than the atomic bombs the US dropped on Hiroshima and Nagasaki in 1945.

The British Ministry of Defence (MoD) has said that Wagner mercenary group’s chief Yevgeny Prigozhin has taken an “unambiguously confrontational” towards the Russian defence ministry.

Yesterday, Prigozhin said that he was expecting a reply from the Russian defence ministry regarding a ‘contract’ of his own drafting which he had delivered to the ministry three days before.

“This follows the MoD’s own ultimatum to Wagner and other ‘volunteer formations’ to sign contracts with the MoD by 01 July 2023,” the British defence ministry said.

It added that although the content of Prigozhin’s document has not been made public, “the act of him delivering it raises the stakes, and is highly likely another deliberate effort to undermine the authority of the official military authorities”.

“Prigozhin’s tone towards the MoD has become unambiguously confrontational. The MoD almost certainly sees this as deeply unfortunate at a time when it is grappling with Ukraine’s counter-offensive,” it added.

India’s stance on Russia in the Ukraine conflict has not faced widespread criticism in the United States, the country’s prime minister Narendra Modi said.

On being asked about critical comments in the US for not taking a more forceful stance against Russia’s invasion of Ukraine, Mr Modi said: “I don’t think this type of perception is widespread in the US.

“I think India’s position is well known and well understood in the entire world. The world has full confidence that India’s top-most priority is peace,” he said.

New Delhi has refused to condemn its old ally Russia for the war in Ukraine and has increased its trade with Moscow to record levels, driven largely by imports of Russian oil.

The Russian forces attempting to stop Ukraine’s advance on the battlefield are planting mines as Moscow looks to fortify territory on the eastern and southern frontlines, the war-hit nation’s commander-in-chief Valerii Zaluzhnyi said.

“The enemy is trying to prevent the advance of our units. For this, [they] deployed a system of fortifications with dense mining of the terrain and a large number of reserves,” he said on Telegram last night.

The Ukrainian commander added that “despite the furious resistance of the occupiers, our soldiers are doing everything possible to liberate Ukrainian territory and the operation continues as planned”.

Zaluzhnyi said he had conducted a meeting with the Ukrainian chief of the general staff of the armed forces, Lieutenant General Serhiy Shaptala, who met troops performing “the most difficult tasks in the areas where the fiercest battles are taking place”.

Russia has claimed that its forces destroyed a Ukrainian stronghold using a remotely-controlled tank filled with a huge amount of explosives, in a purported new war tactic.

The Russian defence ministry said it packed “about 3.5 tons of TNT and 5 FAB-100 bombs” into the tank.

Military bloggers have also flagged this latest Russian war trick where its forces have delivered a “suicide tank” laden with explosives that is likely remote-controlled, alongside visuals of an ancient T-54/55 VBIED tank being blown to smithereens.

FAB-100 bombs on average can carry a 100kg (220-pound) payload.

The video shows the Soviet-era tank touching a mine and exploding before reaching its target near the war’s frontline.

The Independent has not verified the authenticity of the video.

Russia fired a widespread overnight air attack on Ukraine targeting the capital and cities in a wide arc spanning east to west as most of the country spent the night with air raid sirens blasting for several hours.

The strike hit “critical infrastructure” in the city of Lviv, sparking fire, said officials from the military administration of the city of about 700,000 people that lies just 70km (43 miles) from the border with Nato member Poland.

No immediate casualties have been reported.

The Russian air raid targeted telecommunication infrastructure and agriculture and farming properties, said Yuri Malashko, head of the military administration of the Zaporizhzhia region in southeast Ukraine.

Kyiv’s military administration said that its air defence systems were engaged in repelling Russia’s drone attack on the capital and the region.

The overall scale of the attack and the full extent of the damage was not immediately clear.

Ukraine has fought back courageously against Mr Putin’s warped bid to restore territory lost to Moscow with the collapse of the Soviet Union and has continued to defy the odds by defending itself against Russian onslaughts with the help of Western military aid.

Thomas Kingsley and Joe Sommerlad report: