Ukraine Battleground: What We Know Now

— -- Russia pressed Ukraine to call a ceasefire in its battle to wrest control of eastern Ukraine from rebels, but Ukraine's leaders responded today with defiance.

“A great war has come, the likes of which Europe has not seen since the Second World War,” Ukrainian Defense Minister Valeriy Heletey said on his Facebook page today.

Heletey's comments implied that Ukraine was entering combat with Russia. Ukraine and NATO allege that Russia has sent troops and armor across the border when it appeared that pro-Russian rebels were losing to Urkaine's military forces.

On another front, Russia's actions have prompted NATO to form a 4,000 troop "spearhead" force to counter any Russian aggression.

Here's what we know so far:

What is going on in Ukraine?

Ukraine's momentum was abruptly reversed last week as the Ukraine military was closing in on the rebels strongholds Donetsk and Luhansk. Since then, the rebels opened up a new front further south seizing the city of Novoazovsk, and menacing the port city of Mariupol.

Officials in Kiev and Western countries allege Russia has sent in troops and equipment to bolster the pro-Russia rebels, but Moscow denies it and rejects suggestions that it can wield influence over the rebels.

How many Russian troops are currently in Ukraine?

Despite the Russian denials, Ukrainian National Security Council spokesman Col. Andriy Lysenko said Monday that "not less than four battalions and tactical groups of the Russian armed forces are active in Ukraine." A battalion consists of about 400 soldiers.

Last week, a rebel commander said there were 4,000 Russian soldier fighting with the rebels. Those soldiers were on vacation, the commander said.

Where have there been clashes?

Clashes between Russian separatists and Ukrainian forces have been reported in Donetsk and Luhansk regions. Luhansk Airport, located in the eastern most corner of the country, has been taken over by separatist forces of the self-proclaimed Luhansk People’s Republic.

Tanks and soldiers from the Russia-backed insurgents have not yet made it to the coastal town of Mariupol, but residents fear that they are next since their port city is along the Azov Coast. The tourist destination of Novoazovsk, also along the coast but closer towards the Russian border, was reportedly seized by the rebel forces late last week.

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Gaining control of the port city would give the pro-Russian forces - and consequently Russian - a land route between Russia and Crimea which would make it easier to transport some of the oil-rich peninsula’s natural resources to mainland Russia.

How serious is this threat?

The Ukrainian defense minister warned that a battle with Russia will lead to “tens of thousands” of deaths.

“A great war has arrived at our doorstep -- the likes of which Europe has not seen since World War II,” Valeriy Geletey wrote in a Facebook post Monday.

So far, around 2,600 people have died in eastern Ukraine since fighting began in April.

Are international forces going to intervene?

NATO has announced the creation of a “spearhead” force of 4,000 troops who will be ready to address Russia’s “aggressive behavior,” NATO Secretary General Anders Fogh Rasmussen said Monday.

The details of the “high-readiness force” will be hammered out this week at a NATO summit being held this week in Wales. The troops will come as part of a rotational basis from Allied countries, which is seen as a way to get around a 1997 agreement with Moscow that states NATO will not base a substantial number of soldiers on the ground in eastern Europe on a permanent basis.

"(This) ensures that we have the right forces and the right equipment in the right place at the right time," Rasmussen said. "Not because NATO wants to attack anyone. But because the dangers and the threats are more present and more visible. And we will do what it takes to defend our allies."