Swedish citizens Peyman Kia, 42, and Payam Kia, 35, were detained last year and had denied all the allegations.

Peyman Kia's lawyer told Reuters his client still denied the accusations and would appeal against the verdict. His brother's lawyer did on Thursday not reply to requests for comment.

"The brothers have jointly and in consultation, without authorisation and to assist Russia and the GRU, acquired, forwarded and shared information the disclosure of which to a foreign power could be detrimental to Sweden's security," Stockholm district court said in a statement.

The pair were convicted of aggravated espionage and the older brother was also found guilty of the unauthorised handling of classified information.

The court sentenced the older brother to life imprisonment and the younger brother to a term of nine years and 10 months.

(Reporting by Anna Ringstrom and Marie Mannes; Editing by Terje Solsvik and Alison Williams)