Britain on Wednesday called on Russia to free detained Ukrainian pilot Nadezhda Savchenko and told Moscow it and NATO would not be intimidated by "saber-rattling" by President Vladimir Putin.
Savchenko, who has become a symbol of resistance in Ukraine to what Kiev and the West see as Russian aggression, is being held in Moscow on charges of aiding the killing of two Russian journalists in east Ukraine last year.
She has been weakened by a hunger strike against what she says are politically motivated charges.
Britain's Minister for Europe David Lidington, in a letter made public on Wednesday, wrote to Savchenko telling her London was pressing Moscow for her release.
"During 12 months of incarceration you have shown immense courage and resilience," Lidington wrote. "The British government calls on Russia to live up to all of its international commitments and to its commitments under the Russian constitution and to release you immediately."
Separately, while on a visit to Estonia, British Defense Minister Michael Fallon criticized Putin, who on Tuesday said Moscow would be adding more than 40 intercontinental ballistic missiles to its nuclear arsenal this year.
"This is clearly saber-rattling clearly designed to provoke, designed to intimidate. It won't do that," Fallon told reporters at the Estonian military air base of Amari, part of NATO's Baltic air policing mission.
"We are prepared to modernise our defenses and do whatever is necessary to reassure as well as to deter," Fallon added, referring to NATO.