Russian opposition politician Kara-Murza being investigated for treason - RIA

FILE PHOTO - Russian opposition leader Vladimir Kara-Murza, vice chairman of Open Russia, testifies before a Senate Appropriations Subcommittee in Washington

LONDON (Reuters) - Russian opposition politician Vladimir Kara-Murza is being investigated for treason, his lawyer was quoted as saying on Thursday, as authorities step up their case against him for speaking out against the war in Ukraine.

State-owned news agency RIA quoted lawyer Vadim Prokhorov as saying a criminal case for treason - which carries a sentence of up to 20 years - had been opened against Kara-Murza in connection with three of his public speeches.

They included an address to the Arizona House of Representatives in which he said President Vladimir Putin was bombing Ukrainian homes, hospitals and schools.

Moscow says it does not deliberately target civilians, but thousands have been killed in Ukraine. On Thursday, at least three died when a Russian missile destroyed an apartment block in the city of Zaporizhzhia.

Kara-Murza, who holds both British and Russian citizenship and was a pallbearer at the 2018 funeral of U.S. Senator John McCain, was a close aide to opposition leader Boris Nemtsov, who was shot dead in central Moscow in 2015.

Twice, in 2015 and 2017, Kara-Murza fell suddenly ill in what he said were poisonings by the Russian security services, on both occasions falling into a coma before eventually recovering. Moscow denied involvement.

Kara-Murza was arrested in April and declared a "foreign agent". He is currently in pre-trial detention on suspicion of spreading false information about the armed forces under new laws passed eight days after the Feb. 24 invasion of Ukraine began.

(Reporting by Reuters)