A Russian-backed rebels and Ukrainian forces traded accusations on Thursday that each had fired across the ceasefire line in eastern Ukraine, raising alarm.
The exchange is coming at a time when Western countries have warned of the possibility of a Russian invasion any day.
Moscow, which has massed more than 100,000 troops near Ukraine, denies it is planning to invade its neighbour and has said this week it is pulling back some troops.
The West disputes that there has been a significant withdrawal, and the United States said thousands more troops were still arriving.
It was not immediately possible to determine whether the reports of shelling along the front line amounted to an escalation, or fit a pattern of ceasefire violations that have been routine throughout a conflict entering its eighth year.
Western countries have repeatedly accused Russia of planning to use an incident in the rebel-held areas to justify an attack, while Moscow, for its part, has accused Kyiv of planning an escalation to try to recapture territory by force.
Contradicting Russia’s assertions that it is pulling back, a senior official in U.S. President Joe Biden’s administration said up to 7,000 more troops had moved to the border in recent days, including some arriving on Wednesday.
Russia’s defence ministry released video it said showed more departing units.
Maxar Technologies, a private U.S. company that has been tracking the buildup, said satellite images showed that while Russia has pulled back some military equipment from near Ukraine, other hardware has arrived.
A Reuters photographer in the town of Kadiivka in Ukraine’s rebel-held Luhansk region heard the sound of some artillery fire from the direction of the line of contact, but was not able to determine the details of the incident.
The separatists accused government forces of opening fire on their territory four times in the past 24 hours. Kyiv accused the rebels of firing shells, including some that struck a kindergarten, causing shell-shock to two civilians.
A diplomatic source said a longstanding monitoring mission from the Organization for Security and Cooperation in Europe had recorded multiple shelling incidents along the line of contact in the early hours of Thursday.
Since a 2015 ceasefire brought an end to major combat in the separatist conflict, the OSCE has typically reported dozens of ceasefire violations each day, with significant shelling or clashes leading to injuries or death several times a month.
The self-proclaimed Luhansk People’s Republic, one of two rebel regions, said Ukrainian forces had used mortars, grenade launchers, and a machine gun in four separate incidents on Thursday.
“Armed forces of Ukraine have crudely violated the ceasefire regime, using heavy weapons, which, according to the Minsk agreements, should be withdrawn,” the separatists said in a statement.
Referring to the rebels, Ukraine’s military said: “With particular cynicism, the Russian occupation troops shelled the village of Stanytsa Lugansk in the Luhansk region.
As a result of the use of heavy artillery weapons by terrorists, shells hit the kindergarten building. According to preliminary data, two civilians received shell shock.”
Russia denies planning an invasion of Ukraine but says it could take unspecified “military-technical” action unless a range of demands is met, including a promise never to admit Kyiv into the NATO alliance.
The West has rejected the main Russian demands but has proposed talks on arms control and other issues.
The United States and Europe have threatened sanctions if Russia invades, which Moscow has largely brushed off.
