Ukrainian forces fear Russia is set to open a new front against tiny state Moldova which has an army of just 3,250 soldiers.
The new strategy comes amid rising tensions in Transnistria - a Russian-controlled region of around 470,000 people in the former USSR state which has declared itself independent.
If Russia took Moldova it would allow Putin to attack Ukraine from the West in the port of Odessa, lengthening the odds against President Zelensky’s forces.
A source told the Times: “We believe the Kremlin has already taken the decision to attack Moldova.
“The fate of Moldova is very crucial. If the Russians start to take control, we will, militarily, be an easier target and the threat to Ukraine will be existential.”
Roughly 1,500 Russian troops are believed to already be in Transnistria after being flown into the main airfield.
Like Ukraine, Moldova is also not a member of defence organisation Nato.
Transnistrian Moldovan Republic was formed in 1992 after a five-month war in which Russian forces assisted the separatists.
Last month the president of Moldova Maia Sandu called for an urgent meeting with her government following reports of multiple explosions in the breakaway area.
The region is close to Odessa - where eight Ukrainians, including a three-month-old baby, were killed in a recent Russian missile strike.
It comes amid fears the region could be dragged into Russia's war with Ukraine.
President Sandu said certain forces in the region were in "favour of war".
Transnistria authorities have said the explosions this week targeted their state security headquarters in Tiraspol, a Soviet-era radio mast that broadcasted Russian news and a military unit in a village outside Tiraspol.
Ukrainian President Volodymyr Zelensky said it was clear Russia was behind the explosions.
He said: "The goal is obvious - to destabilise the situation in the region, to threaten Moldova. They show that if Moldova supports Ukraine, there will be certain steps."
Ukraine's foreign ministry condemned what it called Russian attempts to drag the region into the war.
It said the incident "coincided with the statements of the Russian military command about their plans to occupy the entire south of Ukraine and establish a land corridor to the Transdniestrian region of Moldova.”
It added: "Ukraine strongly supports the territorial integrity of Moldova in its internationally recognised borders, condemns attempts to involve the Transnistria region of Moldova in Russia’s … full-scale invasion of Ukraine, and calls for a de-escalation of the situation."
Rustam Minnekaev claims the Russian president has ambitions to extend his invasion beyond the borders of the war-torn country.
Russia’s ambassador in Moldova was summoned on Friday after he claimed Moscow was trying to create a route towards a breakaway part of the country where Russian troops are based..
Minnekaev said: “Control over the south of Ukraine is another way out to Transnistria, where there are cases of Russian-speaking people being oppressed.”
