OTTAWA -- A joint Canada-U.S. program that allows so-called "trusted travellers" to more easily cross the border is catching on with Canadian frequent flyers -- but some major airlines aren't keeping pace.

Applications for Nexus cards have increased about 17 per cent since Canada and the United States began to more aggressively push the program earlier this year, causing some delays in processing applications.

The Nexus cards are designed to make border security more efficient by allowing passengers who aren't considered a danger to go through customs more quickly, while border guards focus on travellers who have the potential to be high risk.

But some air carriers don't appear to be training their check-in staff to recognize the cards.

And one carrier, Air Canada, is insisting that passengers travelling to and from the United States carry a valid passport, regardless of whether they have "trusted traveller" status.

The company's website says: "When travelling with Air Canada, all passengers, including Canadian and U.S. citizens, are required to present a valid passport when travelling by air between Canada and the United States, even with a Nexus card."