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Labour leader Sir Keir Starmer has said his party "stands steadfast in solidarity with Ukraine", as he visits Estonia.
He met Nato troops stationed in the country, which also shares a border with Russia, during a trip marking 15 days since the invasion of Ukraine.
Sir Keir also held talks with Estonian Defence Minister Kalle Laanet.
His visit comes as the Army says a small number of soldiers have disobeyed orders and gone to Ukraine to fight.
Estonia, which has a 183-mile (294km) border with Russia, is a full Nato member, but President Alar Karis has warned that Mr Putin's government could use hybrid warfare, including cyber attacks, against his people.
Sir Keir and shadow defence secretary John Healey met Colonel Dai Bevan, who commands Nato Enhanced Forward Presence in Estonia, at the Tapa base in the north of the country.
They will have lunch with troops there and return to the capital Talinn to speak to MPs at the Estonian Parliament.
While travelling from the defence ministry building to Tapa, Sir Keir tweeted: "Today, John Healey and I are in Estonia, to meet with our allies and British troops.
"Labour stands steadfast in solidarity with Ukraine, and we are committed to supporting our Nato allies."
The British Army said a small number of soldiers had "disobeyed orders and gone absent without leave" and may have travelled to Ukraine "in a personal capacity".
There is no suggestion those involved were from the Tapa base.
Defence Secretary Ben Wallace has warned soldiers could face prosecution for desertion if they travel to Ukraine to fight the Russian army.
Sir Keir, while supporting the UK's military stance on Ukraine, has called on the government to do more to help refugees.