Russia and the U.S. have traded accusations amid one of the worst spikes in violence between pro-Moscow separatist rebels and government troops in Ukraine since a cease-fire six months ago.
Ukrainian forces said Wednesday that separatists attacked their positions 82 times overnight, including using artillery banned under the shaky February truce brokered by European leaders.
"The situation is still most tense in the area of the militant-controlled city of Donetsk," the military's so-called Anti-Terrorist Operation (ATO) said on Facebook, according to a translation by Ukraine's Unian news agency.
The statement came after more than 100 explosions, "both incoming and outgoing," were heard on Monday night and early Tuesday in the rebel-held city of Donetsk by Organization for Security and Cooperation in Europe monitors.
Dozens of houses were damaged in the government-controlled town of Sartana, according to the OSCE, which also analysed almost a dozen craters caused by artillery rounds. There have been reports of several deaths, both civilian and military, related to the renewed fighting.
The escalation, which began over the weekend, prompted the State Department to warn on Monday of the "sharp increase in attacks by combined Russian-separatist forces across the ceasefire line in eastern Ukraine."
Spokesman John Kirby told reporters: "There can be no mistake about who is responsible: Russia and the separatists are launching these attacks."
Russian President Vladimir Putin laid the blame at the Ukrainian government. "Regrettably, we are now seeing this conflict escalation," he told reporters in Crimea on Tuesday, according to Reuters. "The blame lies not with the Donbas militia but with the rival side. I hope there won't be full-scale direct clashes."
More than 6,800 civilians and military personnel have been killed in the conflict since April 2014, according to the United Nations. The violence between rebels, who want closer ties with Russia, and the government, which is backed by the West, started after the ouster of Moscow backed President Viktor Yanukovych following violent demonstrations in Kiev.
The U.S. and its Western allies claim Russia has supplied the rebels with troops, training and resources, a charge denied by Moscow.