MOSCOW — The simmering standoff in eastern Ukraine exploded into warfare early Tuesday, pushing the conflict to a dangerous new phase and prompting Russian President Vladimir Putin to warn again that he reserves the right to use force to defend Russian-speaking citizens.
Government forces unleashed ground assaults and air bombardments throughout the region, including heavy artillery shelling around the rebel-controlled city of Slovyansk and pitched battles for control of administrative buildings in Donetsk. The attacks began shortly after President Petro Poroshenko declared an end to a 10-day cease-fire and ordered government forces to renew their effort to quash a pro-Russian separatist insurrection in the east.
The recharged government offensive followed two days of conference calls with the leaders of Russia, France and Germany that failed to yield concrete steps toward a peace agreement, and came as Poroshenko accused Putin of talking about peace while arming and encouraging the separatists.
Ukraine has been in turmoil since December, when former President Viktor Yanukovych spurned a trade deal with the European Union in favor of a $15 billion loan and other concessions from Russia. He was driven from office in the ensuing demonstrations, and after Putin seized Crimea in March the separatist rebellion began brewing in the east, which has historically close ties with Russia.
While President Barack Obama and European leaders have joined Ukraine in accusing Moscow of creating and sustaining the rebellion in eastern Ukraine, Putin blamed the West for the troubles there and talked about protecting Russians outside the country's borders.
"This country will continue to actively defend the rights of Russians, our compatriots abroad, using the entire range of available means — from political and economic to operations under international humanitarian law and the right of self-defense," Putin said.
There were reports of heavy shooting and bombardments all across eastern Ukraine on Tuesday, including civilian casualties. Gunfire cracked in the center of the regional capital of Donetsk, where rebels and government forces fought for control of the Interior Ministry headquarters and a police building came under attack by the separatists.
In the city of Kramatorsk, to the north, four people were killed when a minibus came under artillery fire, Ukrainian news agencies reported.
Ukrainian forces retook control of a checkpoint at Dolzhanksy, in the Lugansk region, one of three important border crossings with Russia that had been seized by rebels. Poroshenko issued a statement congratulating his troops for reclaiming the border checkpoint.