Russian President Vladimir Putin has condemned Ukraine for ending a truce with separatist rebels in the east.
He said his Ukrainian counterpart Petro Poroshenko must now bear direct responsibility for the conflict.
Mr Poroshenko blamed "criminal elements" as he ended the ceasefire and launched a full-scale military operation using aircraft and artillery.
Four people were killed in a minibus when shells hit the rebel-held town of Kramatorsk in Donetsk region.
In other fighting
- A policeman was reported killed in the city of Donetsk when separatist fighters tried to seize the interior ministry's regional headquarters.
- A tank battle between rebels and government forces was reported near the town of Karlivka
- Ukrainian forces said they had regained control of the Dovzhanskyy crossing on the Russian border
At the scene: Oleg Boldyrev, BBC Russian, Kramatorsk
We have seen the results of the overnight and morning shelling here. Lots of building have been damaged; some have balconies missing.
Locals had time to hide in basements before the shelling began. There is no word of the casualties overnight. However, following a second wave of shelling early in the morning, a minibus was hit and at least four people died.
We can still hear shelling going on now. Some locals say there have been more casualties.
Mr Putin said he and European leaders had tried but failed to keep Ukraine on a path to peace.
Speaking to Russian ambassadors in Moscow, Mr Putin said: "Unfortunately, President Poroshenko has decided to resume military action.
"Neither myself nor our colleagues from Europe have been able to convince him that war cannot lead to well-established and lasting peace."
A four-way teleconference on Monday between Mr Poroshenko, Mr Putin, French President Francois Hollande and German Chancellor Angela Merkel had raised hopes the truce would be renewed.
But Mr Poroshenko said in an emotional late night televised address on Monday: "The decision not to continue the ceasefire is our answer to terrorists, militants and marauders." He accused the separatists of multiple violations of Ukraine's unilaterally called ceasefire.
Mr Putin countered: "Up until now, President Poroshenko was not directly involved with orders to start military action. He has now assumed this responsibility fully - not only militarily, but also politically."
Mr Putin vowed he would continue to defend ethnic Russians abroad, using all means available from humanitarian aid to "self-defence".
Mr Putin also said the West was using the Ukraine crisis to destabilise the whole region as part of a policy to "contain" Russia.
He said Ukraine's affairs were an internal matter.
"We need some kind of insurance net around all of Europe so that the Iraqi, Libyan, Syrian - and unfortunately we have to mention the Ukrainian - scenarios do not become a contagious disease," Mr Putin said.
Russia's foreign ministry said Ukraine would "have to answer for crimes against peaceful civilians".
Analysis by Steve Rosenberg, BBC News, Moscow
Every two years, Russian ambassadors fly to Moscow to hear their president set out his foreign policy objectives and describe his view of international affairs.
From what he said, the world according to Vladimir Putin is one of growing "unpredictability", where the "potential for conflict is rising" and where "elementary norms of decency are being ignored".
There are those in the West who argue that President Putin bears some responsibility for this, following Russia's annexation of Crimea. What's more, the EU and the US have accused Moscow of fanning separatist flames in eastern Ukraine.
But President Putin doesn't accept the blame. He criticised the US and Nato. He accused Ukraine of shunning peace. He said Russia preferred co-operation to confrontation. But he warned that Russia would "energetically defend the rights of Russians and compatriots abroad": a message to the West that Russia will take the path of confrontation if it believes it has no choice.
In his address, Mr Poroshenko insisted: "Peace has been and will remain my main goal. Only the means to achieve it have changed.
"We will attack, we will free our land."
Ukraine and some Western powers accuse Russia of arming the separatists - a claim Russia denies.
The trigger for the current crisis was whether Ukraine should lean more towards Russia or the EU.
The refusal of Mr Poroshenko's predecessor, Viktor Yanukovych, to sign an EU trade deal late last year - under pressure from Russia - led to protests in Kiev and his eventual overthrow.
Russia has since annexed Ukraine's Crimea region, and separatists in the east declared independence from Ukraine.
President Poroshenko signed a landmark EU trade pact last Friday.