Twelve other Ukrainian service members were injured in the fighting in separatist areas of eastern Ukraine, the country's National Security and Defense Council said.
The violence happened despite a ceasefire that was agreed to in mid-February for Ukrainian forces and rebels who hold territory in eastern Ukraine's Donetsk and Luhansk regions, near the border with Russia
It's an extension of a conflict that the United Nations says has killed more than 6,000 people since last year and forced 1.6 million people from their homes, including more than 600,000 who sought refuge in other countries.
Militants fired on Ukrainian forces more than 40 times Tuesday night in the Donetsk and Luhansk areas, according to the Ukrainian news agency Ukrinform.
That included artillery, mortars, tanks and small arms fire on Ukrainian positions in the Donetsk region, Ukrinform reported.
The Ukrinform report did not detail what led to Tuesday's fighting or how the Ukrainian forces responded.
Separatists and Ukrainian forces fought for months in the two regions last year and early this year, a conflict that led to souring relations between the West and Russia.
Western leaders and Kiev accused Russia of fostering the conflict by providing weapons and training to the pro-Russian separatists, as well as sending regular Russian troops over the border to fight. The United States and the European Union
levied sanctions against Russia, which denied the allegations.
Unrest in Ukraine began with protests in the country's capital in 2013 after pro-Moscow President Viktor Yanukovych, favoring closer ties to Russia, dropped plans to sign a political and economic agreement with the European Union.
After months of protests and days of deadly clashes between demonstrators and security personnel in Kiev, Parliament ousted Yanukovych in February 2014. Weeks later, Russia annexed Ukraine's Crimean Peninsula.
In April 2014, violence broke out in Donetsk and Luhansk as separatist leaders declared independence from the government in Kiev. Months of fighting followed, and after a September ceasefire quickly fell apart, another truce deal was reached in February following Ukraine-Russia talks mediated by leaders of France and Germany.
Violence continued for weeks after that February deal, however, with hundreds of people reported killed in post-truce violence before both sides withdrew heavy weapons from front lines in Donetsk in March