US airlines are canceling flights to and from Hong Kong following a new rule that would require all crew members to be tested for COVID-19 on arrival.

American had planned to resume flights from Dallas to Hong Kong this week, but pushed the start date back to August 5. United began flying between San Francisco and Hong Kong on Monday, but suspended flights scheduled through Friday while it looks into the new testing requirement.

Hong Kong had previously exempted flight crews from a stringent 14-day quarantine requirement for people entering the city. The city also limits who may enter from overseas, but cargo demand and transit passengers could help airlines make up for low passenger load factors.

Under the new testing requirement, pilots and flight attendants are required to submit to saliva testing on arrival at Hong Kong International Airport. If crewmembers tested positive or were quarantined, it could create logistical difficulties for airlines seeking to position crewmembers and aircraft.

In a statement, American said it was continuously assessing its network and destinations.

"We've been making regular adjustments to our schedule to match demand, and we consider a range of factors including travel restrictions or entry requirements in making network decisions."

As airlines seek to resume international travel during the COVID-19 pandemic, the new testing requirements come as just the latest challenge. The European Union's ban on American travelers , as well as strict entry restrictions in other countries, have led to most of the modest uptick in travel demand being for domestic itineraries.

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