The leaders of the two major pro-Russian separatist groups in eastern Ukraine announced on Tuesday that they are banning their militias from opening fire against the Ukrainian military, Russian state news agency TASS reported.
The unilateral measure will go into effect at midnight on Wednesday, said Igor Plotnitsky of the self-proclaimed Luhansk People's Republic.
"It will be strictly forbidden for any unit of the people's militia of the Luhansk republic to open fire in response to shooting or any provocation from the side of the Ukrainian army and national guard," Plotnitsky said.
Alexander Zakharchenko of the self-proclaimed Donetsk People's Republic said the measure was being put in place to support a long-standing ceasefire agreement that has so far largely failed.
"We will support the Minsk agreements until the very end," Zakharchenko said.
Three Ukrainian servicemen were killed and 15 wounded during fighting with pro-Russian rebels in the country's east in the past 24 hours, a spokesman for Ukraine's military said Tuesday.
Another soldier went missing in action, spokesman Olexandr Motuzyanyk said in comments carried by Ukrainian state news agency Ukrinform.
The Luhansk People's Republic told TASS that the Ukrainian military attacked rebel positions twice over the past 24 hours. Rebels in the neighbouring Donetsk region also reported fighting.
Tensions between Russia and Ukraine have escalated in recent weeks, with both countries putting troops on full combat alert after Russia accused Ukrainian forces of planning attacks in the Crimea region.
Russia occupied and annexed Crimea from Ukraine two years ago in response to Kiev ousting its pro-Russian president amid mass protests calling for closer ties with the West.
Russia has since supported the simmering separatist rebellion in Ukraine's two easternmost regions, but has denied allegations that it has provided weapons and troops for the revolt.
More than 9,000 people have been killed in the conflict, according to estimates by the United Nations.