The FSB detained two 19-year-olds in the city of Saransk, in the Mordovia republic, central Russia, saying they had been planning to fight for Ukraine.

Russian news agencies quoted the FSB as saying that the two had "planned to go to Ukraine through the territory of a third country in order to join combat operations against the Russian army."

A criminal case on "participating in a terrorist organisation" was opened against them.

The FSB also detained two people in the Siberian city of Tomsk for treason, accusing them of sending money to Kyiv's army.

The pair had "committed state treason in the form of giving financial help to a foreign state in an act aimed at harming the security of the Russian Federation", it said.

A treason case was opened against them.

Russia's FSB security service regularly reports arrests of people accused of working for Ukraine, where Moscow has been fighting for more than two years.

Moscow also put exiled feminist Zalina Marshenkulova on a wanted list.

Her name was added to a list of wanted people on the Russian interior ministry's website.

Russian media reported that a "justifying terrorism" case had been opened against her this week for social media posts on the death of anti-Ukraine military blogger Vladlen Tatarsky.

Tatarsky was blown up by a statuette handed to him in Saint Petersburg a year ago.

Marshenkulova had said she had no sympathy "for an aggressor and a former robber who supported the murders in Ukraine".

In a post on Tuesday, she said a criminal case had been opened against her for "calling a bandit a bandit."

Russia has cracked down on dissent and on any support for Ukraine inside the country since it began its military operation there more than two years ago.