Russia's window to invade the Ukraine is shortening as a change in weather could turn border lands into a swamp.
Melting snow and increased spring temperatures mean the border between Russia and the Ukraine could turn swampy - making a smooth invasion more difficult.
It means the next month could see an increase in tensions to the point of "maximum danger", according to experts.
James Sherr, an associate fellow of the Russia and Eurasia program at Chatham House, told the UK House of Commons Defence Committee: "If you go beyond February, the logistical challenges of mounting a considerable and devastating attack become progressively less favourable to Russia.
"So, the next month is going to be the point of maximum danger."
The warnings come as Russia reportedly moves military medical units to frontline in ominous sign Ukraine war is close.
This shows Vladimir Putin is moving to a level of readiness for war that he has lacked in previous military buildups, The Wall Street Journal reports.
Sightings of medical units have been absent among an avalanche of social media videos of a vast Russian massing of troops and equipment around Ukraine’s borders, which has escalated in recent hours.
But the Journal cited Western defence officials regarding the medical units with claims the move amounts to the final preparation for an invasion.
While their presence at the frontline does not mean a Russian attack on Ukraine is certain, military doctors and field hospitals are seen as a prerequisite for battle.
Separately, it is known Russia recently held a major medical training exercise involving army medics and 1,200 military police officers in its Western military district which borders Ukraine.
This involved drills on troops with mock “shrapnel and bullet wounds”.
Russian army medics and police rehearsed evacuating the wounded from battlefields - and "operating on severely wounded" men, said a military statement.
Military police officers under the supervision of military doctors performed resuscitation procedures, both on simulated sick servicemen and on special mannequins, stated an Interfax report.
It read: “Military police officers in conditions of simulated combat performed evacuation of wounded servicemen, reception and screening at a medical facility, provision of first aid to the injured, as well as performing surgeries to save critically wounded people.”
Until now, the absence of intelligence about medical units has led Ukrainians to believe that an attack by Russia was not an immediate threat.
