Reports from eastern Ukraine say government forces and separatist rebels were engaged in combat early on May 16, with government forces firing artillery at separatist positions on the outskirts of Slovyansk.
A rebel spokesman said fighting that began on May 15 was continuing in the early morning hours of May 16 on the approaches to Slovyansk near the villages of Andreyevka and Semyonovka, and at the towns of Krasny Liman and Mount Karagun.
Mortar rounds and heavy machine guns were being used in the overnight battle.
Meanwhile, an ultimatum issued by Sergei Zdrilyuk, a self-styled separatist commander in Donetsk, was due to expire later on May 16.
Zdrilyuk said on May 15 that all Ukrainian armed forces had 24 hours to withdraw their armored vehicles and check points from around ”Donetsk, Mariupol, and other cities” controlled by separatists.
If those forces do not withdraw, Zdrilyuk said: “I will have enough manpower and resources, and my commander gave me his support in this matter, for all of this to be destroyed and burnt down.”
In Kyiv, acting President Oleksandr Turchynov claimed battlefield successes on May 15.
He said government security forces had destroyed a base used by pro-Russian separatists near Slovyansk and gained control of a five-kilometer zone around a television tower south of Kramatorsk that had been seized by separatists.
Turchynov also said the TV tower near Kramatorsk has begun transmitting Ukrainian channels again.
The separatists who seized the tower had changed its signals so that it had been broadcasting only Russian state TV while under their control.
Ukraine's Defense Ministry says government forces also destroyed a rebel hideout outside of Kramatorsk on May 15, capturing three "terrorists."
The ministry said there were no casualties among its soldiers.
The fighting comes after the government in Kyiv on May 14 launched a "national dialogue" aimed at de-escalating Ukraine’s crisis.
Pro-Russian separatists from eastern Ukraine were not represented at those talks.
Officials in Kyiv say they will not talk to the separatists until they lay down their arms.
The roundtable talks are based on an OSCE road map to de-escalate the crisis ahead of Ukraine's May 25 presidential election.
NATO Secretary-General Anders Fogh Rasmussen, speaking at a security conference in Bratislava on May 15, criticized Russia for not taking steps to de-escalate the crisis.
U.S. Secretary of State John Kerry said in London that Russia would face “sectoral sanctions” – economic sanctions against entire sectors of the Russian economy – if the Kremlin tries to disrupt Ukraine's May 25 presidential election.
Based on reporting by Reuters, AP, AFP, and ITAR-TASS