Kyiv and its Western allies said they feared that Russia might be trying to create a pretext to unleash war on Thursday, after reports of shelling across the front line in Ukraine’s longstanding conflict with Moscow-backed separatists.

Ukraine and the pro-Russian rebels gave conflicting accounts of Thursday’s shelling and the details could not be established independently. The reports from both sides suggested an incident more serious than the ceasefire violations that are regularly reported on the line of contact in eastern Ukraine’s Donbass.

Ukraine’s President Volodymyr Zelenskiy said the pro-Russian forces had shelled a kindergarten, in what he called a “big provocation”. For its part, the Kremlin said Moscow was “seriously concerned” about reports of an escalation after the separatists accused government forces of opening fire on their territory four times in the past 24 hours.

British Foreign Secretary Liz Truss tweeted: “Reports of alleged abnormal military activity by Ukraine in Donbass are a blatant attempt by the Russian government to fabricate pretexts for invasion. This is straight out of the Kremlin playbook.”

NATO was “concerned that Russia is trying to stage a pretext for an armed attack against Ukraine. There is still no clarity, no certainty about the Russian intentions”, the Western military alliance’s Secretary General Jens Stoltenberg said.

“They have enough troops, enough capabilities to launch a full-fledged invasion of Ukraine with very little or no warning time,” he told reporters at the alliance’s headquarters in Brussels. “That is what makes the situation so dangerous.”

Moscow denies Western accusations it is planning to invade its neighbour and said this week it was pulling back some of the more than 100,000 troops it has sent to the area. The West disputes that there has been a significant withdrawal and the United States said thousands more troops were still arriving.

A senior Ukrainian government source said the shelling at the line of contact with Russian-backed separatist forces went beyond the scale of ceasefire violations routinely reported throughout the conflict.

“It is not typical. It looks a lot like a provocation,” the source told Reuters.

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.

Kyiv accused the rebels of firing shells at several locations, including some that struck a kindergarten and others that hit a school where pupils had to flee to the cellar.

Video footage released by Ukrainian police showed a hole through a brick wall in a room scattered with debris and children’s toys. Separate images showed emergency workers escorting small children and teachers from a building.

Kremlin spokesperson Dmitry Peskov said Moscow had already warned that a concentration of additional Ukrainian forces near the Donbass frontline created a risk of provocations. Kyiv has denied massing extra troops in the area.

Contradicting Russia’s assertions that it has been 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.

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Russia’s defence ministry released video it said showed more departing units. Maxar Technologies, a private U.S. company that has been tracking the build-up, said satellite images showed that, while Russia has pulled back some military equipment from near Ukraine, other hardware has arrived.

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, often minor incidents of test-firing weapons. Reports of significant shelling or clashes that lead to injuries or death can occur several time 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 are 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, a threat which Moscow has largely brushed off.