WestJet Airline warned Wednesday about a "difficult" second quarter as ongoing economic weakness put a damper on air travel.

President and CEO Sean Durfy said the airline's revenue per available seat mile (RASM) -- a key measure to compare costs across carriers -- is showing "significant declines" in the second quarter, with an expected drop of between 16 and 18 percent compared with last year.

"A continued weakened economy and aggressive pricing are leading to reduced fare levels, making it a difficult quarter," Durfy said in a statement.

"While the second-quarter results will undoubtedly be weaker than previously expected, we believe that investors will look beyond the near-term revenue challenges facing the Canadian airlines," Versant Partners analyst Cameron Doerksen said.

WestJet, a no-frills carrier, is currently one of the few profitable airlines in North America.

"While these challenges are comparable with what the North American airline industry is experiencing, we believe we are better positioned than most to return to stronger RASM results when the economy begins to rebound," Durfy said.

WestJet also said its planes flew less full last month as its seat capacity rose and the number of paying passengers fell.

In May, WestJet's load factor dropped 5.4 percentage points to 74.1 percent from 79.5 percent in May 2008.

Traffic declined 5.8 percent year-over-year to 1.06 billion revenue passenger miles while the airline's seat capacity, measured in available seat miles, rose 1.2 percent to 1.4 billion over the same period.