Six Ukrainian soldiers were killed on April 5 in the east of the country in two separate incidents as the conflict in eastern Ukraine entered its second year.
The Interior Ministry said a military vehicle was hit as it drove across a bridge in Schastye, a government-held town about 170 kilometers northeast of the city of separatist-held Donetsk.
The ministry accused Russian-backed separatists of firing the shell, but that report could not be independently confirmed.
It said initial reports showed "militants had fired a laser-guided antitank missile."
Ukraine's antiterrorism center has identified all four of the servicemen, adding that they came from the Kharkiv region.
The information center for the so-called Luhansk People's Republic denied reports its forces were involved.
The pro-Russian separatists claimed the Ukrainian military vehicle hit a landmine placed on the bridge by Ukrainian forces.
In a separate incident less than an hour later, a Ukrainian military vehicle hit a landmine near the town of Shyrokyne, east of the city of Mariupol, killing two servicemen and wounding one other.
The previous day, the government reported the deaths of another three soldiers when a mine exploded near the town of Avdiyivka, a government-held town north of Donetsk.
The fatalities were the first announced by the authorities in almost a week after the death of a Ukrainian army soldier reported on March 30.
The deaths have underscored the fragility of a cease-fire in force since February, when it was brokered by leaders from Ukraine, Russia, Germany, and France.
Both sides in conflict have accused the other of violations since the cease-fire went into effect.
Some 6,000 people have been killed since fighting broke out between pro-Russian separatists and the Ukrainian government a year ago.