An Ontario father whose two sons were allegedly abducted by their non-custodial mother says he had fears that something might happen well before they disappeared more than two years ago.

Stephen Watkins said he wrote letters to the government and pressed to have his ex-wife's passport and driver's licence revoked after she threatened to run off with their children amid a bitter custody battle.

"There were lots of signs and clues that told me there was a chance that they could be abducted," Watkins told CTV's Canada AM on Tuesday. "Going through court when there is parental alienation happening is a clue. And obviously where there is a threat – I received a threat that I would never see my boys again – that is another clue."

Watkins last saw his children on March 6, 2009; Alexander was seven years old and Christopher was four.

Police believe they were abducted to Europe by their non-custodial mother Edyta Watkins. York Regional Police say her father drove her and her sons across the border into the U.S., where they boarded a plane from Detroit to Frankfurt, Germany.

Det. Const. Jesse Mann said they believe she has since moved her boys to Poland, her country of origin, but need proof before local authorities will come on board.

"It is difficult to determine how engaged the Polish authorities are because all the information goes through the appropriate channels, RCMP and Interpol," Mann said.

A Canada-wide arrest warrant has been issued for Edyta Watkins, who goes by several other names, and she also appears on the RCMP's most wanted list.

Watkins said he became fearful something might happen after his ex-wife began taking the children to Polish language classes and refused to turn over their passports.

Watkins said the issue has exposed a major security issue. Her passport had been cancelled after she failed to pay child support, but she still managed to cross the U.S. border and fly to Europe.

At the time, Canadian citizens could cross a U.S. land border with only a driver's licence and birth certificate.

Watkins said his ex-wife suffered from post-partum depression and became abusive toward him after giving birth to their first child. He has also said she was abusive toward the children.

Watkins said the court had granted him custody of the children, allowing his ex-wife to visit on weekends.

"It is so difficult, you can't imagine, not knowing where they are or if they are alive. We see all these things in the news, there are tons of parental abductions, but we know bad things are happening. Finding them is really important," Watkins said.

With files from The Canadian Press