Britain on Thursday said the attack on a kindergarten in Ukraine was a “false flag operation designed to discredit the Ukrainians.”

“We fear very much that that is a thing we will see more of over the next few days,” Boris Johnson said.

Johnson’s remarks came as the UK, US and NATO warned that Russia would try to stage a false “pretext” for the invasion of Ukraine.

Speaking in Lincolnshire, Johnson said: "I wish I could give everybody better news about this, but I have to tell you that the picture is continuing to be very grim.

"Today, as I'm sure you've already picked up, a kindergarten was shelled in what we are taking to be - well, we know - was a false flag operation designed to discredit the Ukrainians, designed to create a pretext, a spurious provocation for Russian action.

"We fear very much that that is the kind of thing we will see more of over the next few days.

"What we are doing is making that we do everything to strengthen the package of sanctions that will follow immediately should there be a Russian invasion."

Ukraine's President Volodymyr Zelenskyy called the incident a “big provocation,” accusing pro-Russian forces of shelling the kindergarten.

In return, Moscow said they were "seriously concerned" of reported escalation as separatists accused Ukrainian forces of opening fire on their territory.

British Foreign Secretary Liz Truss also accused Russia of a "blatant attempt" after reports of kindergarten shelling.

"Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion," Truss tweeted.

"This is straight out of the Kremlin playbook," she wrote.

Both Ukraine and separatists in Donbas accuse each other of shelling in eastern Ukraine.

Kremlin spokesperson Dmitry Peskov said the "situation near the borders of Russia can ignite at any moment."