Airlines are stepping up cleaning and hygiene measures, requiring face masks and even checking passengers' temperatures prior to boarding in some cases in an effort to ensure safety and provide travelers with added peace of mind amid the COVID-19 pandemic. However, some carriers are also limiting capacity and blocking seats to allow for physical distancing on board. Here are the airlines keeping some seats empty this summer.

Alaska Airlines, the carrier handing out yellow cards to passengers who violate its mask policy, recently expanded its Next-Level Care safety measures, including blocking middle seats to cap capacity at 65 percent through July 31. Families or large groups traveling together can request to sit together by contacting the airline's reservations team.

Delta is blocking middle seats on larger aircraft and aisle seats on smaller planes through September 30 and capping cabin seating at 60 percent in Main Cabin and 50 percent in First Class. "Confidence in a safe travel experience is key to a successful recovery," said Joe Esposito, Senior Vice President - Network Planning, in a statement last month. "While we're rebuilding our network at home and abroad, it's even more critical that we provide the highest industry standard of safety, space and clean so when our customers are ready to travel, we're ready for them."

Ultra-low-cost carrier Frontier, which is the only U.S. airline requiring passengers to undergo a temperature screening prior to boarding, is blocking a limited number of seats, about 20 per flight through August 31.

Hawaiian Airlines, which is set to resume most of its mainland U.S. routes next month, is blocking seats to limit capacity to roughly 70 percent this summer. The policy remains in effect through August 31.

JetBlue's seat distancing policy was scheduled to expire on July 6 but the carrier extended it through the end of July. The airline is blocking middle seats on its Airbus aircraft and aisle seats and on its smaller Embraer 190 planes. "We're known for generous legroom and space, and now more than ever, those choosing to travel want as much space as possible," said Joanna Geraghty, JetBlue's president and chief operating officer, in a statement back in May. "We are pleased to extend our efforts to keep seats free and help everyone onboard spread out."

Southwest is keeping middle seats open through at least September 30. However, customers can still pick their own seats, so travel companions can sit together if they choose. The low-cost carrier is also boarding in groups of 10 to allow for physical-distancing.