Russia is reported to be sending army medical units to the Ukrainian front, according to Western defence officials.
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.
New videos show yet more heavy military equipment heading both for Belarus and the borders of rebel-held eastern Ukraine.
They also show that trains are still being loaded in the far east of Russia for a 6,000-mile transfer via the Trans-Siberian Railway.
A consignment of tanks and infantry fighting vehicles was spotted at Smolensk, presumably heading to Belarus.
Meanwhile, new footage shows bombing drills in Kaliningrad, close to NATO states Lithuania and Poland, involving Su-30SM and Su-24 warplanes.
More view shows Russian riflemen training in polar conditions for winter warfare in an unspecified northern location.
In the increasingly tense Black Sea, major war games involved the Admiral Essen frigate, the Ladny patrol ship, the Naberezhnye Chelny and R-60 missile boats in live-fire drills against mock air and sea “enemies”.
In total more than 30 warships were involved in the latest high intensity drills by Russia’s naval forces.
In the Urals, troops exercised with Russia’s mobile S-400 Triumf surface-to-air missile system in a joint exercise with Su-34 multirole fighter-bombers and Su-24MR reconnaissance aircraft.
“The S-400 crews simulated the targeting and destruction of a simulated enemy's planes and helicopters after the aerial targets entered the impact zone,” said the country’s Central military district.
