KIEV - President Petro Poroshenko warned Friday that the threat of terror was "significantly" rising across Ukraine as pro-Russian militias resumed attacks that killed two more federal servicemen.

The Western-backed leader said Ukraine's efforts to stamp out the 15-month eastern revolt had drained security resources and left peaceful cities open to attacks from criminals and those allied to rebel fighters.

"The terrorist threat level has significantly risen outside the zone where we are conducting our anti-terrorist operation," the 49-year-old former chocolate magnate said in a televised address.

Ukraine refers to the insurgents as "terrorists" - a label that infuriates Russia.

Moscow denies backing the fighters and defends the uprising as an expression of free will in a mostly Russian-speaking region that looks sceptically at the pro-European stance adopted by the former Soviet nation in the past year.

The fighting itself resumed with renewed vigour Friday after a one-day lull.

"The enemy is once again resorting to the use of heavy weapons, sabotage and reconnaissance groups," Ukrainian military spokesman Andriy Lysenko told reporters.

Ukrainian officials reported the death of two soldiers in separate incidents.

One of the servicemen died in a brief battle that broke out 15 kilometres (nine miles) northeast of Lugansk - capital of the smaller of the two separatist regions.

"One serviceman was killed in fighting and another was injured and hospitalised," Kiev-appointed Lugansk region leader Gennadiy Moskal.

Lysenko said another soldier was killed and nine wounded in various exchanges of fire whose details he failed to disclose.

More than 6,500 people have been killed since the conflict broke out in the wake of the February 2014 ouster of a Russian-backed leader and his replacement by a strongly pro-Western leadership.

A February truce deal managed to contain some of the clashes but daily exchanges of fire continue to add to the death toll.