A major battle flared Wednesday outside a main separatist stronghold in war-torn eastern Ukraine, leaving more than a dozen dead and threatening to push the country back into full-blown civil war.
Each side blamed the other for the new violence, which Ukrainian officials said began near Donetsk when about 1,000 pro-Russian rebel fighters, backed by artillery and tanks, launched a pre-dawn attack west of the city.
Rebels reported 15 civilian and combatant deaths, while authorities in Kyiv said at least three government soldiers were killed and 30 others were wounded in the town of Marinka.
Ukraine's defense minister said the attempt by rebels to take Maryinka had been thwarted. Senior rebel commander Eduard Basurin said separatist forces had only used weapons in defense, denying any attempt to advance.
"We are not carrying out offensive actions," separatist press service DAN quoted him as saying.
Ukraine Prime Minister Arseniy Yatsenyuk accused Russia of inciting the new fighting, saying Moscow had ordered militants to begin military operations near Donetsk, some 50 kilometers from the Russian border.
The Kremlin, which insists it has no direct role in the crisis and has repeatedly denied supporting the rebellion with arms and fighters, took the side of the separatists.
"In Moscow, we are following very closely, and are deeply concerned by, the provocative actions by the Ukrainian armed forces," Kremlin spokesman Dmitry Peskov told journalists in a conference call.
Washington blamed Moscow for the violence. "Russia bears direct responsibility for preventing these attacks and implementing a cease-fire. Any attempts to seize additional Ukrainian territory will be met with increased costs," State Department spokeswoman Marie Harf told reporters.
Wednesday's fighting came one week after a Reuters correspondent published an eyewitness account of a huge Russian military buildup near the Ukraine border. The report cited troops and hundreds of pieces of weaponry marshaled on Russian soil at a makeshift firing range 50 kilometers from the Ukraine border.
It also said many Russian vehicles had been stripped of identifying markings and that many of the troops had removed insignias from their fatigues.
The report was widely seen as corroborating ongoing NATO claims that Russia, despite a February cease-fire, continues to provide hardware and personnel support to rebels.
NATO chief Jens Stoltenberg, speaking in May, described the Russian support as "a steady flow of heavy equipment, tanks, artillery, ammunition, air defense systems and lots of training" to separatists.
The truce signed in February by Russia, Ukraine, France and Germany required both sides to withdraw heavy weapons from the line of contact. But international observers say that provision has been routinely violated.
More than 6,400 people have been killed since April 2014, when separatists launched the rebellion against Kyiv in Russian-speaking eastern Ukraine.
Some information for this report came from Reuters.