The Ukrainian military and Russian-backed separatists accuse each other of launching offensives in the government-held industrial town of Avdiyivka and firing heavy artillery in defiance of the two-year-old Minsk ceasefire deal.
Eight Ukrainian troops have been killed and 26 wounded since fighting intensified on Sunday - the heaviest losses for the military since mid-December, according to government figures.
"The current escalation in Donbass is a clear indication of Russia's continued blatant disregard of its commitments under the Minsk agreements with a view of preventing the stabilisation of the situation," Ukraine's foreign ministry said in a statement.
The peace deal was agreed in February 2015, but international security monitors report ceasefire violations on a daily basis, including regular gun and mortar fire.
The latest clashes mark the first significant escalation in Ukraine since the inauguration of U.S. President Donald Trump, whose call for better relations with Moscow has alarmed Kiev while the conflict remains unresolved.
Ukrainian authorities said they were prepared for a possible evacuation of Avdiyivka's 16,000 residents, many of whom have little or no access to utilities after shelling hit supply infrastructure.
The International Committee of the Red Cross's Ukraine delegation tweeted that there was no water, electricity or heating in the town and the temperature was -18 degrees Centigrade (0 Fahrenheit). "Hostilities continue and people start to lose hope," it added.
Meanwhile Russia's Foreign Ministry said Ukrainian government troops had launched deadly offensives on rebel positions and warned that the region was "on the verge of humanitarian and ecological catastrophe".
Earlier, Kremlin spokesman Dmitry Peskov accused the Ukrainian authorities of organising the attacks as a ruse to try to distract attention from domestic and other problems.
Close to 10,000 people have been killed since fighting between Ukrainian troops and rebels seeking independence from Kiev erupted in April 2014.
Ukraine and NATO accuse the Kremlin of supporting the rebels with troops and weapons. The United States and European Union have imposed sanctions on Russia over the conflict, as well as for its annexation of Crimea.