Canadian Prime Minister Mark Carney warned Thursday that the United States is trying to break Canada so it can “own us” as he urged oil-rich Alberta not to begin a process toward separation in Monday’s referendum.

The vote is not a direct referendum on independence. Alberta, which has the world’s fourth-largest proven oil reserves, will decide whether to remain a province or instruct the government to begin the legal process toward a future referendum on separation.

“This referendum comes at the worst possible time,” Carney told a large pro-Canada rally in Calgary. “The world has become more dangerous and divided. Some outside our borders are working to break us so they can own us.”