In March Russian parliament gave Putin permission to intervene in eastern Ukraine with his military if people who speak Russian in eastern or southern Ukraine were in danger and last week he expressed the hope he didn't have to do it.
However, today the Ukraine has opened the attack on pro-Russian rebels in the east of the country and there have been severe clashes, for example near Slavjansk, where at least 5 "terrorists" have been killed by the Ukrainian army
Meanwhile, Putin has responded and stated that this would be a severe crime against their own people and there will be consequences
Source: AFP
Update:
5 of 24 Ukrainian oblasts (best to be compared to provinces or states) have recently declared independence, though it's not legal and thus not binding.

One of those five oblasts, Donetsk, has officially declared war on Ukraine after their attack on the pro-Russian rebels earlier today.
Meanwhile, Ukraine has been spreading folders that people need to stay away from Government buildings in Slaviansk, where they started their attacks earlier today.
Quote:
Mitch Potter ✔ @MPwrites
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Wow: this just fell from sky in #Slaviansk. 1000s of pro-#Ukraine leaflets warning 'Don't come near govt buildings.'
3:38 PM - 24 Apr 2014
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Sources: Twitter (as can be seen) + several papers and news outlets worldwide