BORIS Johnson has accused Russia of a “false flag” attack to create a pretext for war in Ukraine.

That assessment has been backed by the Nato secretary general Jens Stoltenberg following reports Russian-backed separatists have clashed with state forces in east Ukraine, with claims that rebel shells hit a nursery school.

It comes as the UK and US warn Russia has deployed 7000 more troops near Ukraine’s borders, despite claims by Moscow that units have started returning to base after military exercises.

Johnson, speaking during a visit to RAF Waddington in Lincolnshire, 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 sure that we do everything to strengthen the package of sanctions that will follow immediately should there be a Russian invasion.”

UK Foreign Secretary Liz Truss, who is in Kyiv for talks with Ukrainian leaders, said she was “very concerned” about the situation and accused the Kremlin of trying to create a pretext for an invasion.

Separatist authorities in the Luhansk region claimed there had been an increase in Ukrainian shelling along the tense line of contact, describing it as a “large-scale provocation” and they then returned fire.

The Kyiv government disputed the claim, saying that separatists had shelled its forces but they did not fire back.

A nursery building in Stanytsia Luhanska was hit, wounding two civilians, according to the Ukrainian military.

Truss – who has warned that Russian President Vladimir Putin could mount a “false flag” operation to justify an invasion – said: “Reports of alleged abnormal military activity by Ukraine in Donbas are a blatant attempt by the Russian government to fabricate pretexts for invasion.

“This is straight out of the Kremlin playbook.”

The National: Prime Minister Boris Johnson sits in a military plane during his visit to Royal Air Force Station Waddington in LincolnshirePrime Minister Boris Johnson sits in a military plane during his visit to Royal Air Force Station Waddington in Lincolnshire

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The UK and US have accused Russia of massing more troops on Ukraine’s borders, totaling around 130,000. The Kremlin has repeatedly denied having any intention of invading.

Truss said she was “very concerned about reports today of increased Russian aggression: over 7000 extra troops near the Ukraine border and an attack by pro-Russian troops on a kindergarten in Ukraine”.

She urged Russia to withdraw its troops as there was “still time for diplomacy and de-escalation”.

Nato secretary general Jens Stoltenberg also claimed Russia is attempting to engage in “false flag operations” to create a pretext for war in Ukraine.

Speaking at a press conference at alliance headquarters in Brussels, he warned there were “many” Russian intelligence officers active in Donbas, east Ukraine.

“Of course this is a concern,” he said. “This is the reason why we are so closely monitoring what is going on.”

Russia released footage in recent days claiming to show troops and equipment moving back to base following military exercises in regions close to the Ukraine border.

But UK Defence Secretary Ben Wallace said nearly 60% of Russia’s land combat forces were still in place around the borders of Ukraine.

Although an invasion is unlikely to result in British troops fighting, as Ukraine is not a Nato member, the military build-up has caused anxiety among eastern members of the alliance.

UK forces have been committed to efforts to bolster defences on Nato’s eastern flank.

Britain has already said it is doubling its number of troops in Estonia, with around 850 members of the Royal Welsh battlegroup heading from Sennelager in Germany and bases in Europe for the Baltic state, while 350 Royal Marines are being despatched to Poland.

Four additional RAF Typhoon jets have landed in Cyprus to join Nato patrols over eastern Europe, while offshore patrol vessel HMS Trent will shortly be joined in the eastern Mediterranean by HMS Diamond, a Type 45 destroyer.