A David-and-Goliath contest is being played out in Black Sea peninsula
Despite Russia's superior manpower, experts say morale remains high in Ukraine's armed forces
Agence France-Presse in Kiev
Ukraine has placed its army on full combat alert, but with ageing equipment and limited personnel, it remains cautiously defensive in the face of an incursion by Russian forces.
Since the Russian parliament gave the green light on Saturday for its armed forces to intervene in Ukraine, thousands of soldiers have been flexing their muscles in Crimea. Their uniforms might be stripped of identification, but they are widely believed to be acting under Kremlin orders.
They have effectively taken control of the most strategic centres of the Black Sea peninsula, blocking the roughly 15,000 Ukrainian soldiers stationed there in their barracks, and seizing key government buildings and airports.
Watch: Ukraine mobilises army after Russia's threat to invade
Kiev has responded by mobilising its reservists, accusing Russia of "declaring war" and starting an "armed invasion".
On the face of it, Ukraine would face a decidedly David-and-Goliath affair should the conflict escalate. Russia has 766,000 active frontline personnel, compared with 160,000 for the Ukraine, of which half are conscripts with ageing equipment. And Moscow already has a large contingent based in the Crimea peninsula, with its Black Sea Fleet - consisting of 25,000 men, 388 warships and 161 aircraft - stationed at Sevastopol.
A former part of the Russian empire and Soviet Union, Crimea is considered a crucial part of Moscow's sphere of influence.
On the other hand, says Matthew Clements, editor of Jane's Intelligence Review, the Ukrainian army has been designed precisely for the type of land-based conventional war that would likely emerge in case of a Russian invasion.
"If the Ukrainian forces remain unified and there are no defections to the Russian side, they have some chance of holding Russia back in a full-combat situation for a considerable time," Clements said.
"Russia can mobilise greater numbers and more modern equipment, but this would be a much more even contest than Russia's war in Georgia [in 2008]."
Ukraine's Soviet-era equipment would struggle to match up to the equipment mobilised by Russia, the world's third-largest spender on defence.
Outnumbered, Kiev has so far ordered its soldiers on the Crimean peninsula to avoid any moves that might offer Russia an excuse for a full-blown invasion.
Ukrainian Prime Minister Arseniy Yatsenyuk on Sunday criticised the "deliberate sabotage" of the country's defence capacities by the previous regime.
But he added: "Despite the sabotage and the catastrophic state of our finances, the government has found it possible to finance the Ukrainian armed forces during this very difficult period."
Having stayed out of the political crisis in recent weeks, army morale remains high despite its lack of resources, according to Valentyn Badrak, director of the Centre for the Army Research, Demilitarisation and Disarmament in Kiev.
"Morale can overcome numbers. If the Ukrainian soldiers show their willingness to resist, they can push Russia back," he said.
Kiev accuses Moscow of airlifting around 6,000 more soldiers into the region on Friday and Saturday, and there are concerns that Moscow will move troops into other parts pro-Russian part of Ukraine in the coming days.
The threat is also fuelling far-right nationalist groups on the Ukrainian side.