Russian opposition figure Vladimir Kara-Murza -- jailed in April for denouncing the Kremlin's Ukraine offensive -- has been charged with high treason, his lawyer told Russian news agencies on Thursday.

"Our client has been charged after speaking out critically against the Russian authorities three times, at public events in Lisbon, Helsinki and Washington. These speeches did not pose any threat (to the country), it was public, open criticism," Kara-Murza's lawyer, Vadim Prokhorov, told the TASS news agency.

The veteran opposition politician had been detained in April on charges of disobeying police orders. His arrest was later extended on charges of "discrediting" the Armed Forces and he remains in detention. 

In July, investigators introduced fresh criminal charges of cooperating with an “undesirable” foreign NGO, which could see him face an additional six years in prison.

A longtime critic of President Vladimir Putin, Kara-Murza survived two poisoning attempts in Moscow in 2015 and 2017, which he claimed were linked to his efforts to lobby the West to sanction Russian officials responsible for human rights abuses.

He is one of the few Russian opposition figures to have stayed in the country following the Kremlin's invasion of Ukraine.

The treason charges against Kara-Murza mark the second high-profile treason case in Russia this year. 

A Moscow court last month sentenced Ivan Safronov, a former defence journalist and later an adviser to Russia’s state space agency, to 22 years in a maximum-security prison for allegedly supplying foreign intelligence services with classified information.