Two soldiers and a pro-Russian rebel were killed in the last 24 hours in eastern Ukraine where isolated clashes continue in breach of a truce to end the yearlong war, officials on both sides said Wednesday.

Two army troops died and four were injured, army spokesman Andriy Lysenko said without giving any details. He also accused separatist rebels of violating a February ceasefire a dozen times during the same period, saying rebels used heavy weapons including mortars.

Two bomb attacks early Tuesday shook Ukraine's second city Kharkiv, near the Russian border and the frontline of the country's war, causing some damage but no casualties.

The Kharkiv prosecutor's office said a probe was underway for a "terrorist attack" after a strong explosion in the city center, which knocked out windows in a nearby university building and setting car alarms screeching.

The young Cossack fighter gripped his automatic rifle as he swore he would defend the town of Perevalsk in war-torn east Ukraine that he and his comrades now control. 

"We're expecting an attack. We're prepared, no one can enter the town," the man in his twenties said, a traditional Cossack fur hat on his head.

Ukrainian President Petro Poroshenko on Monday raised the idea of holding a referendum on the decentralization of the country's cities but was swiftly given the cold-shoulder by pro-Russian rebels.

"I'm ready to launch a referendum if you decide it is necessary," Poroshenko said at the first meeting of a constitutional commission set up to carry out political reforms required under February's Minsk 2 accords.

Maria Gladchenko stood by the smoldering ruins of her home after the conflict in east Ukraine that began a year ago once again tore through her neighborhood.

"There's nothing for us to do now but start our lives over," the 60-year-old retired school teacher sighed, pointing to the burnt-out remnants filled with the stench of fire.

Czech President Milos Zeman and the U.S. ambassador to Prague have clashed over the former's plan to travel to Moscow for the World War II victory anniversary, an event largely snubbed by western leaders.

In a TV interview last week, U.S. ambassador Andrew Schapiro questioned the staunchly pro-Russian Zeman's wish to be "probably the only EU head of state" to attend the May 9 showcase parade on Moscow's Red Square.

Six Ukrainian troops were killed in two separate incidents in the troubled separatist-held east Sunday as isolated clashes continue to violate a fragile ceasefire to end the year-long war, officials said.

"Unfortunately, six Ukrainian soldiers have been killed today," army spokesman Oleksandr Motuzyanyk told AFP.

A year since the start of the fighting in eastern Ukraine, Vladimir Putin may not have emerged the winner in his showdown with the West but he has not lost either, analysts say.

By supporting Ukrainian separatists, they say, he took a huge risk but it largely paid off as it allowed him to punish Kiev's pro-Western authorities for seeking to turn their back on Russia and stand up to the West.

Three soldiers died when a mine exploded in Ukraine's separatist-held east where isolated clashes continue to undermine a ceasefire aimed at ending a year-long war.

Army spokesman Andriy Lysenko said Saturday that "as a result of the explosion of an enemy mine ... three Ukrainian soldiers were killed, two injured" in the last 24 hours.

Ukraine security services on Friday announced the launch of a "terrorism" probe after the explosion of a homemade bomb outside the Kiev offices of Russia's state-controlled Sberbank the previous day.

The explosion, which caused no injuries but shattered nearby windows, was the first of its kind in the Ukrainian capital.