The defence secretary has warned that eastern Europe’s security is under threat as he revealed at least 100 more British troops will likely be sent to Poland to help tackle a migrant crisis on the Belarus border.

Ben Wallace he was “deeply concerned” about an unusual build-up of Russian forces around Ukraine, which neighbours both Poland and close Kremlin-ally, Belarus.

Speaking on a trip to Poland on Thursday, he would not be drawn on whether the two flashpoints were linked.

But the senior minister said: “There is lots of activity in this part of Europe that is threatening security and I think one of our responses is to show that solidarity, both politically and indeed militarily.”

One way the UK has done this is with the deployment last week of a small group of Royal Engineers to the Polish border to look at how to help strengthen a fence preventing migrants – bussed to the frontier by the Belarusian regime – from crossing over.

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Mr Wallace said he would read an assessment from their reconnaissance trip, but he appeared ready to send a larger team to assist the Polish military.

“It is likely that a squadron of [Royal] Engineers will be sent here to support them to make sure that the border is properly secure and show them this kind of hybrid activity from Belarus won’t be tolerated and won’t work,” he said.

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The defence secretary also announced he had reached an agreement with his Polish counterpart to work together on building a £3bn missile defence shield for Poland.

The development of the Ground-Based Air Defence System could well be viewed as a provocation by Russia.

Mr Wallace denied this was the case.

“Britain and Poland have had a long and deep relationship for 200 years,” he said.

“We are natural and strong partners, we work together.”

The defence secretary, echoing a statement released by the G7 group of rich democratic nations, accused Belarusian leader Alexander Lukashenko of exploiting the desperation of people to escape war-torn countries like Iraq to attack the European Union in a form of hybrid warfare, under the threshold of conventional war.

He said the regime was flying migrants to Belarus and then bussing them to its borders with the European Union – Poland, Lithuania and Latvia – to create friction within the EU, where migration is a hugely sensitive issue.

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“This is a horrendous activity on the backs of some of the most vulnerable people, young women and children, being forced almost over these borders and that again adds to a destabilising and growing anxiousness in the West, which is something that always leads to insecurity,” Mr Wallace told Sky News.

He was speaking while visiting some 150 British troops that are already based in Poland as part of a 1,000-strong, US-led battlegroup.

The so-called Enhanced Forward Presence (EFP) is one of four such battlegroups, including in Estonia, Latvia and Lithuania, which was set up by the NATO alliance to deter Russian aggression in the wake of its annexation of Crimea in 2014.

Mr Wallace watched as the British soldiers took part in a live-firing exercise with their Polish, US and Romanian allies, charging around a large field in armoured vehicles firing on a fictional enemy.

The aim of the four battlegroups is to deter Russia from attacking a NATO country.

But concern is growing that President Vladimir Putin might be planning a new offensive against Ukraine, which is not within the alliance.

Asked whether Russia might be on the brink of an invasion of Ukraine, Mr Wallace said: “What we see is a worrying build-up of troops in the region.

“I think really the message from Europe, the message from the G7, the message from the UK is that all nations engaging in such destabilising activity should cease and that is not the way forward.”

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