Ukraine has placed its army on full combat alert, but with ageing equipment and limited personnel, it has remained 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 flexed their muscles in Crimea -- their uniforms stripped of identification but 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.

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 an army of around 845,000 soldiers, compared with just 130,000 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)."

- Deliberate sabotage -

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.

Ukraine's new 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 that led to Yanukovych's downfall, army morale remains high despite its lack of resources, said 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.

Pravy Sektor (Right Sector), a far-right group that was on the front lines of the anti-government protests in Kiev, has already threatened to get involved.

"We will procure weapons to prepare to confront Russian occupation forces," its spokesman said Sunday.

The group claims the mantle of the controversial Ukraine Insurgent Army (UPA), the nationalist paramilitary movement that fought for the country's independence in the Second World War -- pitching itself against Soviet, Polish and Nazi forces -- and which was only disbanded by the Soviet army in the 1950s.