Soldiers who were found to be absent without leave could have left the country to fight in the war on a “personal capacity”, the army added.
“We are actively and strongly encouraging them to return to the UK,” a spokesperson added.
Service personnel are forbidden to travel to Ukraine until further notice.
During a Thursday press conference, Liz Truss clarified her comments after she said would “absolutely” back UK nationals who wanted to go fight in the Ukraine war.
The Foreign Secretary Liz Truss insisted she was just “expressing support for the Ukrainian cause”.
Speaking at a press conference in Washington, she said: “I have been very clear that the travel advice from the United Kingdom is not to go to Ukraine.
“There are better ways of helping the efforts in Ukraine, namely donating to the DEC (Disasters Emergency Committee) appeal.
“What I said the other week was expressing support for the Ukrainian cause, they are fighting a just cause and we are doing all we can to support them.”
Previously, the Foreign Secretary replied “absolutely” when she was asked whether she backed anyone from the UK who wanted to volunteer to help the Ukrainians fight.
She said on BBC One’s Sunday Morning programme: “That is something people can make their own decisions about.
“The people of Ukraine are fighting for freedom and democracy, not just for Ukraine, but for the whole of Europe. Absolutely, if people want to support that struggle, I would support them in doing that.”
