The UK is to deploy additional troops to Poland and Lithuania amid tensions at their borders with Belarus.
The Ministry of Defence (MOD) announced that 140 military engineers will be sent to Poland, adding that a separate team has also been sent to Lithuania.
It follows the deployment of a "small team" of UK personnel to the Polish border with Belarus last month.
"Poland and Lithuania, along with their Baltic neighbour Latvia, have been under significant pressure from migration originating from Belarus and facilitated by the Lukashenko regime for a number of months," the MOD said.
The deployment comes amid increased tensions with Russia over its military presence near the Ukrainian border.
Foreign Secretary Liz Truss said on Wednesday that it would be a "strategic mistake" for Russia to ramp up aggression against Ukraine.
She added that Russia must "respect the sovereignty and the territorial integrity of Ukraine".
The 140 troops from the Royal Engineers will arrive in Poland from the end of December, with the deployment expected to last until April.
The MOD said they will work with the Polish military at the border with Belarus, including providing "infrastructure support".
The troops are in addition to the 150 British personnel already based in Poland as part of NATO's enhanced forward presence.
In Lithuania, a reconnaissance team of British troops will "establish if the UK can offer any expertise or capabilities to help counter current pressures", the MOD added.
Defence Secretary Ben Wallace said: "Our commitment to European security is unwavering and we will always offer support to our allies.
"This non-combat support will assist Polish efforts to protect their border and pass on vital engineering expertise."