Authorities in Ukraine said security forces suffered dozens of casualties in attacks in the eastern part of the country, the deadliest of which targeted a roadblock set up by government forces near the town of Volnovakha, in the Donetsk region.
Ukraine's Health Ministry said 17 soldiers died in the attacks and at least 36 were hospitalized with injuries.
The ministry said 16 soldiers died in the Volnovakha attack, in which grenade-launchers and mortar shells were used against the roadblock in the early morning hours.
The Defense Ministry earlier said one soldier was killed in an overnight attack in the neighboring region of Luhansk.
The Donetsk attack was the deadliest for Ukraine's military since it launched an offensive six weeks ago against pro-Russian separatists who have declared independent "people's republics" in the two eastern regions.
The attacks come three days before Ukraine holds a crucial presidential election on May 25.
Separatists in both Donetsk and Luhansk have vowed to prevent voting from going ahead in those regions.
Also on May 22, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of trying to escalate the conflict ahead of the vote.
Yatsenyuk called on the UN Security Council
to hold an urgent meeting on the crisis.
NATO Secretary-General Anders Fogh Rasmussen meanwhile said that limited Russian troop movements near the border with Ukraine "may suggest" preparations for a withdrawal.
NATO's top military commander also said NATO has noted some movement of Russian troops away from the Ukrainian border, but that it was "too early" to assess their size or importance.
Supreme Allied Commander in Europe U.S. General Philip Breedlove said that a "very large and very capable force" still remains near the border.
Ukrainian officials said that even if the Russian forces were moving away from the border, "armed terrorists" were still infiltrating Ukraine.
Russian President Vladimir Putin rejects the legitimacy of the pro-Western Ukrainian government, but he says he ordered the withdrawal to help create "favorable conditions" for the May 25 election.
Wolfgang Ischinger, the mediator for a peace plan for Ukraine sponsored by the Organization for Security and Cooperation in Europe (OSCE), has urged both government forces and the separatists to refrain from violence in the run-up to the election.
Ukraine's Central Election Commission said on May 22 that separatists were blocking the work of election officials in 16 of the 34 electoral districts in the Donetsk and Luhansk regions.
With reporting by Reuters, AFP, UNIAN