Accusations of heightened fighting were traded Thursday between Russia-backed separatists in eastern Luhansk and Ukrainian authorities, following the U.S. warning that Moscow is looking for a pretext to invade.

Ukraine has been in conflict with rebels in the east of the country since 2014, in a war that has cost thousands of lives, but the latest reports have intensified fears of Russian intervention.

The Ukrainian military's command center in the east alleged that Russian-backed forces "with special cynicism" had fired heavy artillery at the village of Stanytsia Luhanska.

"The shells hit a kindergarten," it said.

"According to preliminary data two civilians were injured. Public infrastructure was also damaged. Half of the settlement was left without electricity."

Ukraine's Foreign Minister Dmytro Kuleba did not mention any injuries but condemned the alleged shelling.

"We call on all partners to swiftly condemn this severe violation of Minsk agreements by Russia amid an already tense security situation," he said, referring to a 2014 international accord supposed to halt the war.

Photographs on the command center's Facebook account showed civilians sheltering in a cellar, a kindergarten playroom with rubble strewn on the floor and a shell hole in the wall.

Separately, Russian news agencies reported that the separatist force in Luhansk has claimed that Ukraine has escalated fighting.

"Over the past 24 hours, the situation on the line of contact has escalated significantly," Yan Leshchenko, head of the People's Militia in the self-declared Luhansk republic, told reporters.

"The enemy, on the direct orders of the Kyiv military-political leadership, is making attempts to escalate the conflict."

In Moscow, Kremlin spokesperson Dmitry Peskov described the allegation of a Ukrainian escalation as "disturbing."

"This is a matter of very deep concern," he said.

"We hope that our opponents from Western capitals, from Washington, from NATO, will use all their influence to warn the Kyiv authorities against further escalation."

Ukraine and its Western allies say Russia has deployed a huge potential invasion force on Ukraine's borders, and U.S. intelligence has accused Moscow of seeking to create a pretext by faking an atrocity.

NATO says more than 100,000 Russian forces are on the border, but Moscow insists it is seeking a diplomatic route to resolve its security concerns and roll back the West's influence in eastern Europe.