The mother of Bradley Manning, the US army private who is this week expected to be handed a long jail sentence for his role in the WikiLeaks dump of US state secrets, has said she believes she may never see him again.

Speaking after the 25-year-old was found guilty of espionage, theft and computer fraud, Susan Manning urged her son to "never give up hope".

Susan Manning, who lives in Pembrokeshire in Wales, has health problems and has not seen her son since 2011. In a rare statement in the Mail on Sunday, she said: "I know I may never see you again but I know you will be free one day. I pray it is soon. I love you Bradley and I always will."

The sentencing phase of Manning's trial began last Wednesday, a day after he was convicted for passing to WikiLeaks more than 250,000 US diplomatic cables and several hundred thousand battlefield logs from the conflicts in Afghanistan and Iraq, many of which were published by the Guardian.

Susan Manning reportedly could not bring herself to watch TV or listen to the radio to hear last week's verdict but waited for her sister, Sharon Staples, to tell her by phone.

Staples told the paper the family is now "praying for leniency" and described her sister's shock at seeing Manning on her last visit to see him locked up in the US marine base in Quantico in February 2011.

"He was sitting on the other side of a glass partition and when I walked in I heard the sound of the chains round his hands and feet before I saw him," Staples said. "Most of the time they sat in silence but held each others' gaze. She didn't get to hug him but was able to tell him she loved him."

Afterwards, according to Staples, she said: "You wouldn't treat a bloody animal like they're treating Bradley."

• This article was updated on 4 August 2013 to correct a reference to the sentencing phase of Manning's trial.