MOSCOW - A prominent commander of Russian-backed separatists in a breakaway area in eastern Ukraine sought to rally his veteran troops on Thursday, heralding a potential intensifying of the region's conflict.

Alexander Khodakovsky, a former political leader in the self-proclaimed Donetsk People's Republic who now oversees a military unit, said that "all who want to rejoin ranks should be ready."

"At previous meetings with veterans of the militia, the algorithm of actions was discussed, and the places of arrival were indicated. I will inform you of the date and time of the gathering by all available means," Khodakovsky wrote on his Telegram account.

Earlier in the day, Ukraine's military and the separatists traded fire in a ceasefire violation. The clash caused concern among U.S.-led NATO countries that Russia may use it as a pretext for an attack on its neighbour.

Russia has amassed up to 150,000 troops and equipment around Ukraine's borders, the United States says. Moscow denies planning to invade. This week it said it was pulling back some troops and on Thursday it said some had returned to bases from Crimea, which it annexed from Ukraine in 2014.

Washington says Russia is not withdrawing, but in fact sending more forces.

Ukraine and pro-Russian rebels gave conflicting accounts of shelling across the front in the Donbass separatist region.

Ukrainian President Volodymyr Zelenskiy said the pro-Russian forces had shelled a kindergarten, in what he called a "big provocation".

The separatists, for their part, accused government forces of opening fire on their territory four times in the past 24 hours.

Neither account could be verified.