Tensions in the east of Ukraine have risen dramatically after Russian-backed separatists opened fire across the line of control with Ukrainian forces, hitting a nursery school and injuring three people. According to the Organisation for Security and Cooperation in Europe (OSCE) there were “multiple shelling incidents” on Thursday morning across the frontline in the Donbas region. The attack on the city of Stanytsia Luhanska blew a hole through the wall of kindergarten number 21. Classes had begun when the building in Depovska Street was reportedly hit by a shell. Video showed debris and masonry strewn over a play area. The pupils were in another room when the missile hit, but a teacher, laundry worker and security guard were left with concussion, according to reports. The Ukrainian military said 32 shells landed on the city, also injuring a serviceman and disrupting electricity supplies. Ukraine’s president, Volodymyr Zelenskiy, accused the Russian side of “provocative shelling” at a time when the US president, Joe Biden, had warned that a Russian invasion of Ukraine remained “distinctly possible”. The secretary general of Nato, Jens Stoltenberg, said he was “concerned that Russia is trying to stage a pretext for an armed attack against Ukraine”. The UK foreign secretary, Liz Truss, echoed this. “Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion,” she tweeted. “This is straight out of the Kremlin playbook.” Ukraine’s foreign minister, Dmytro Kuleba, tweeted: “Civilian infrastructure damaged. We call on all partners to swiftly condemn this severe violation of Minsk agreements by Russia amid an already tense security situation.” Maria Mezentseva, a Ukrainian MP, said: “I see it as a further provocation because the previous ones didn’t work. We see them going after social infrastructure. The idea is to provoke the Ukrainian side into a reaction.” The OSCE logs regular firing between Moscow-armed separatists and Ukrainian soldiers. In recent months this low-level conflict has been relatively calm as Russia has moved 150,000 troops to Ukraine’s borders, and Ukrainian soldiers have been under instruction not to return fire. Russia, for its part, has accused Kyiv of trying to provoke an escalation to recapture rebel territory by force. It says Ukraine is guilty of genocide, which Ukraine strongly denies. The Kremlin said on Thursday it was deeply concerned by the flare-up in violence in eastern Ukraine and hoped that the west would use its influence on Kyiv to prevent further escalation. The Kremlin spokesperson Dmitry Peskov dismissed reports of a planned invasion by Russia but said Moscow was watching the situation closely. He said Russia had started pulling back some of its troops that had completed drills in areas adjacent to Ukraine, but that the process would take time. Putin’s intentions remain unclear, after inaccurate predictions by Washington that an invasion might begin on Wednesday. Belarus’s president, Alexander Lukashenko, said some Russian equipment and ammunition would remain in his country once joint military exercises with Russia, taking place next to Ukraine, concluded at the weekend. The US, UK, and Stoltenberg have all expressed deep scepticism that Russia is pulling its forces out. The Kremlin said time was needed for troops to return to their bases, saying it took weeks for them to deploy for military drills. On Wednesday, the Russian Duma asked Putin to recognise the two separatist political entities in the Donbas – the Luhansk and Donetsk people’s republics – as independent states. The Kremlin has not decided how to respond. Formal recognition would dramatically raise tensions and kill off the Minsk agreement, signed by Kyiv and Moscow in 2015. Russia has repeatedly accused Zelenskiy’s pro-western government of failing to implement the accords. On Thursday, the Luhansk people’s republic claimed it had come under repeated attack. Ukrainian forces had used mortars, grenade launchers and a machine gun in four separate incidents on Thursday, it asserted. “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. Ukraine’s military denied the claim and said “Russian occupation troops” had cynically targeted civilians. “As a result of the use of heavy artillery weapons by terrorists, shells hit the kindergarten building. Two civilians received shell shock,” it said. The Guardian has seen video apparently showing shelling by separatists against the frontline city of Hirske, controlled by Ukraine, in Luhansk Oblast. A series of percussive booms can be heard. On Wednesday Nato said Russian military capability was only increasing in numbers and strength, despite claims by Moscow it was partially withdrawing its forces.