Seven Ukrainian soldiers were killed in clashes initiated by pro-Russian separatists in the last two days, according to Kiev's military.
The casualties represented a spike in violence in Ukraine's east, which has been embroiled in conflict since Russia first annexed the Crimean Peninsula in early 2014.
Reuters reported that, according to Ukrainian officials, separatists starting attacking government positions on Sunday in the eastern town of Avdiyivka. The violence resulted in the deaths of five Ukrainian troops on Sunday and two more on Monday; nine soldiers were also wounded in clashes on Sunday.
"The situation in the Avdiyivka industrial zone is challenging. The enemy continues to fire at our positions with heavy artillery and mortars," Oleksandr Motuzyanyk, a spokesman for Ukraine's military, told reporters on Monday.
The last spike in violence was reported in mid-December, leading the State Department to express concerns about casualties resulting from a "Russian-separatist attempt to seize additional Ukrainian territory" in areas near Donetsk. At the time, it represented the highest casualty rate in 18 months.
Fighting between pro-Russian rebels and Ukrainian forces has persisted for nearly three years despite international efforts to broker a ceasefire agreement. The United States and European Union have imposed sanctions on Russian individuals and entities for their involvement in the conflict in Ukraine.
Earlier this month, Ukraine filed a lawsuit against Russia in the United Nations' highest court, accusing the Russian Federation of financing acts of terrorism and engaging in discrimination through its intervention in eastern Ukraine, thereby violating international law.
On Saturday, President Donald Trump spoke by phone with Russian President Vladimir Putin for the first time since his inauguration. Ahead of the call, Trump was warned by Republicans and Democrats against lifting sanctions on Russia over the Ukraine conflict, after a White House aide indicated that the two might broach the subject.
Trump and Putin discussed cooperating to fight terrorism and improving relations but did not talk about the U.S. lifting sanctions on Russia, according to White House and Kremlin officials.
Trump has indicated he would like to pursue warmer ties with Russia.