Ukrainian military spokesman Andriy Lysenko said the troops had died in a mortar attack on the government-held village of Mayorsk around 35 kilometres north of the rebels' de facto capital Donetsk.
He said five government soldiers were also wounded over the past 24 hours in separate incidents along the 500-kilometre front splitting the pro-Russian industrial regions of Lugansk and Donetsk from the rest of Ukraine.
These and other kinds of heavy weapons are prohibited under the peace agreements aimed at ending the two-year conflict.
A series of truce agreements have helped reduce the violence, although sporadic clashes continue and prevent the sides from reaching a firm political reconciliation deal.
Kiev and the West accuse Russia of supporting the insurgents and sending regular troops across the border, claims that Moscow denies.