MONTREAL (Reuters) -A shortfall of air traffic controllers in Canada is at times forcing airport towers at Kelowna and Winnipeg airports to close when the one controller on duty goes on a break, according to a memo to Air Canada ( ACDVF ) pilots seen by Reuters.

Air Canada ( ACDVF ) advised its pilots to avoid the two airports in the provinces of British Columbia and Manitoba when staffing shortages are severe enough to shut their air traffic control towers, the October 14 memo said.

It was unclear how often the airports have been closed and their representatives did not immediately respond to requests for comment.

The memo said NAV CANADA, which manages the country's civil air navigation system, has advised airlines that air traffic control shortages will occur at Winnipeg and Kelowna, "but this could expand to other airports."

NAV CANADA said in a statement on Thursday that closures in Winnipeg and Kelowna are planned for "low traffic periods to minimize impact" and are being taken to ensure safe operations and employees' well-being. It did not provide details of normal staffing levels at those towers.

The closures are the latest instance of a North American shortfall of air traffic controllers leading to potential flight delays and added costs for carriers. In Canada, affected flights would need additional fuel, for example, as they might need to stay in a holding pattern or divert due to a controller staffing shortage closure, the memo said.

Air traffic control staffing is of particular concern in the United States due to a government shutdown, with the Federal Aviation Administration saying this week that the problem has delayed travel at several airports.

Nick von Schoenberg, president of the union which represents Canadian controllers, CATCA, said the Canadian airport closures were broader than just Winnipeg and Kelowna and were intended to provide fatigue-related breaks for controllers. They "indicate a lack of resilience in the system due to staffing challenges," he said.

Tim Perry, president of the Air Line Pilots Association in Canada, which represents Air Canada ( ACDVF ) pilots, said the lack of stability due to shortages must be addressed.

"Air traffic controller shortages continue leading to delays at various airports across Canada," he said.