An air raid alert has been declared for every one of Ukraine's regions at the same time, in the latest sign that Russia is preparing for a major attack. 

This morning, media outlet Nexta announced on Twitter that there had been an "an air-raid alert on the entire territory of Ukraine". 

The Kyiv Independent also announced that an air raid alert had been declared "in all of Ukraine's regions at once". 

Ukrainian MP Lesia Vasylenko revealed multiple explosions had occurred in Kyiv. The air raid siren had sounded for almost an hour. 

"3 explosions in #Kyiv right now. One after the other," she wrote on Twitter. 

"The air raid warning has been on for an hour. Most likely #putin gone livid because of the #Moskva sinking. Oh well, we'll just keep standing to #Russia's annoyance." 

The alert follows warnings that President Vladimir Putin could launch an "immediate escalation" of his war in Ukraine after his warship the Mosvka was struck in an apparent Ukrainian missile attack. 

Russia's Black Sea flagship, which has been involved in the naval assault on Ukraine, has been "seriously damaged" by an explosion, Ukrainian state media reported on Thursday, as Moscow threatened to strike Kyiv's command centres. 

Kyiv claims the ship has "started sinking" and hundreds of the Moskva's 510-strong crew may have been killed. 

It was unclear exactly what caused the explosion on the Moskva missile cruiser or the extent of the damage, as both sides gave conflicting reports. 

Russia's defence ministry was quoted as saying the damage was caused by ammunition detonating "as a result of a fire", adding that the cause of the blaze was being investigated. 

But a spokesman for the Odessa military administration, Sergey Bratchuk, said on Telegram that "according to available data, the cause of the 'serious damage' was Neptune domestic cruise missiles". 

Russia's defence ministry said the crew had been evacuated, but anti-Russian politician Ilya Ponomarev said only 50 – less than 10 per cent – of the crew had been rescued so far. 

The governor of Odessa also said Ukrainian forces had hit and sunk the vessel with missile strikes, but Oleksiy Arestovych, adviser to Ukrainian President Volodymyr Zelensky said "we don't understand what happened". 

"If Ukraine has really sunk the Mosvka, expect immediate escalation from Moscow," Russia analyst Clint Ehrlich wrote on Twitter. 

"There will be a political imperative for the Kremlin to push this setback from the headlines with positive news. We may see large-scale strategic bombing, something Russia has held back on to date." 

Putin could unleash bombers 

Last month, US intelligence experts told Newsweek that, contrary to widespread perception in the West, Putin appeared to be holding back from unleashing the full power of his bombers. 

Despite the shocking scenes of "massive" destruction, they cautioned that the Russian leader's conduct to date did not suggest his goal was maximising civilian damage. 

"We need to understand Russia's actual conduct," a retired Air Force officer, now working as an analyst with a military contractor advising the Pentagon, told the publication. 

"If we merely convince ourselves that Russia is bombing indiscriminately, or [that] it is failing to inflict more harm because its personnel are not up to the task or because it is technically inept, then we are not seeing the real conflict." 

A senior Defence Intelligence Agency analyst also argued much of the devastation seen in cities across Ukraine was the result of a contested ground war involving peer opponents. 

"The heart of Kyiv has barely been touched," he told Newsweek. "And almost all of the long-range strikes have been aimed at military targets." 

The DIA analyst added, "I know it's hard … to swallow that the carnage and destruction could be much worse than it is. But that's what the facts show. This suggests to me, at least, that Putin is not intentionally attacking civilians, that perhaps he is mindful that he needs to limit damage in order to leave an out for negotiations." 

The experts said the Russian Air Force had not launched any large-scale, methodical bombing campaign; air and missile strikes were almost exclusively in support of ground forces. 

"Think of the Russian Air force as flying artillery," the retired Air Force officer said. "It's not an independent arm. It has undertaken no strategic air campaign as American observers might be used to from the last 30 years of American conflict."