CHICAGO (Reuters) -Frontier Airlines CEO Barry Biffle said on Tuesday the budget carrier is seeing an opportunity to grow its market share, thanks to financial troubles at rival Spirit Airlines and Southwest Airlines' decision to end its policy of allowing free checked bags.

To cash in on the opportunity, the Denver-based carrier has announced 20 routes it plans to start this winter. It is also rolling out a host of offers to cultivate loyal customers and poach passengers from rival airlines.

The measures are aimed at making Frontier the number one low-fare carrier in the top 20 U.S. metro areas, Biffle said in an interview.

"There are ... things that are taking place that have kind of opened the door for Frontier," he told Reuters.

Southwest's decision to start charging for bags has created a "level playing field" in the low-fare airline industry as customers are now choosing airlines primarily on price, Biffle said.

Meanwhile, Spirit's future remains uncertain as its finances remain shaky just months after emerging from bankruptcy. Earlier this month, the company warned that it might not continue to operate if its financial results failed to improve rapidly.

The financial troubles have forced Spirit to shrink operations, enabling Frontier to swoop in some of its key markets like Fort Lauderdale in Florida. Beginning in November, Frontier will start service to Fort Lauderdale from Detroit, Dallas, Houston, Chicago, and Charlotte.

"ULCC capacity is going down. And we think we can fill that void," Biffle said.

The airline, however, would adjust capacity on some of its existing routes to serve the new routes, he added.

Frontier, like other U.S. carriers, has been grappling with soft travel demand in the domestic market. The company has forecast a wider-than-expected loss for the current quarter and has been betting on network changes, capacity cuts and improved product offerings to lift its earnings.

On Monday, it unveiled loyalty matching programs to grab customers from other airlines. Biffle said Frontier is aiming to double its loyalty revenue to $6 per passenger by next year.