A Ukrainian soldier was killed and three were wounded in fighting between government forces and separatists in eastern Ukraine, the armed forces said Saturday.
On a visit to Kiev, US diplomat Victoria Nuland urged the sides to respect the terms of a February ceasefire agreement reached in Minsk.
"It's now important that all sides walk the walk, not just talk the talk,"said Nuland, the assistant secretary of state for European and Eurasian affairs.
Military spokesman Oleksandr Motuzyanyk told a press briefing: "The situation in the anti-terrorist operation zone remains tense. The armed rebels regularly breach the ceasefire."
He also charged that separatist "bandits" were building up "military equipment, ammunition and manpower along the frontline" while "the Ukrainian soldiers are strengthening their line of defence."
Pro-Russian separatists have in turn reported violations of the ceasefire by Ukrainian forces.
"In 24 hours, we have recorded no less than 19 incidents of firing by Ukrainian forces on the territory of the people's republic of Donetsk," the "defence minister" of the breakaway region was quoted as saying by its official agency.
The Ukrainian army has been battling pro-Russian separatists for more than a year in the country's east, in a conflict in which more than 6,100 people have been killed.
A truce that remains fragile was agreed in February in Minsk at talks mediated by France and Germany and attended by Russian President Vladimir Putin.
Speaking in Kiev, Nuland insisted on the necessity of meeting all the terms of the truce and said the United States was ready to become more involved in the process.
"We are eager to deepen our involvement in helping the parties achieve the full implementation," she said.
Nuland, who met President Petro Poroshenko on Friday, said she was leaving Ukraine "with a renewed sense of optimism."
"We know that ... reforms are painful, but you are on a right course to a more democratic, cleaner, more European country," she said.