Britain will send hundreds more troops close to Russia's border, the Government has said, as the Prime Minister also called for "pressure" on Moscow over the Syria crisis.
Around 800 soldiers along with tanks, armoured vehicles and drones will now head to Estonia in the spring in a Nato effort to reassure the Baltic states over Russian aggression.
The boosted mission, up from 500 announced earlier this year, will be Britain's largest long-term deployment to one of Russia's neighbours since the end of the Cold War.
Defence sources said the six-month deployment to Tapa army base, around 100 miles (160 km) from the border, was the start of a persistent UK presence in the country.
The British troops will form one of four Nato battalions being deployed in response to a perceived threat from Russia to the alliance's eastern allies.
The announcement came as Russia abandoned plans to refuel and provision warships headed for the war in Syria at a Spanish port following international pressure on Madrid.
Russia's embassy in Spain said on Wednesday (local time) that vessels from a squadron led by the Admiral Kuznetsov, Russia's only aircraft carrier, would not stop as planned for refuelling in Ceuta, a Spanish port on the North African coast.
"The Russian Embassy in Madrid has just informed us that it is withdrawing the request for permission for stopovers for these ships and these stopovers have therefore been cancelled," the Spanish Foreign Ministry said.
The Russian defence ministry denied ever planning to stop at Ceuta and said the carrier group carried sufficient fuel and supplies to operate "autonomously" through out their mission.
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"The Russian ministry of defence never sent any such requests to Spain about the aircraft carrier Admiral Kuznetsov visiting Ceuta for refuelling," said Maj Gen Igor Konashenkov, a military spokesman.
Eight ships from Russia's northern fleet, including the Admiral Kuznetsov and the Peter the Great, a missile-carrying battle cruiser, passed through the English Channel en route to Syria last week.
The group is expected to join a flotilla operating off the coast of Syria, where the Kuznetsov's MiG-29k and SU-33 fighters may be used to take part in bombing raids over Aleppo.
Theresa May said "What we have seen, sadly, is that the Russians are already able to unleash attacks on innocent civilians in Syria.
"What matters is that we put pressure on Russia to do what everybody agrees is the only way that we are going to resolve this issue, which is to ensure that we have a political transition in Syria, and that's where we should focus our attention."
Sir Michael Fallon, the Defence Secretary, said the battle to liberate the Syrian city of Raqqa, the capital of Islamic State of Iraq and the Levant's so-called caliphate, will begin within weeks.
Meanwhile, concerns were growing about the apparent deployment of two powerful Russian missile corvettes to the Baltic.
The Zeleny Dol and the Serpukhov, which carry long-range nuclear-capable Kalibr cruise missiles, are to join Russia's Baltic fleet in Kaliningrad, Izvestia, a pro-government broadsheet, reported.
The Buyan-M class corvettes pose "a much more serious threat to Nato than an aircraft carrier," said Alexander Mozgovoi, the editor of National Defence, a Russian military journal.
"They are not big and expensive, but they pack a serious punch. From the southern Baltic they could hit London. In fact their Kalibr missiles can cover most of Europe from there," he said.