As airlines look to the end of the first quarter, they face two emerging questions.

One concerns the impact of capacity increases. The other is whether negative international perceptions of President Trump's proposed travel ban could lead to diminished travel to the U.S. from abroad.

Regarding capacity, United (UAL) - Get Report disclosed last week that it will boost full-year 2017 consolidated capacity growth to between 2.5% and 3.5%, up from previous guidance between 1% and 2%. United's domestic capacity will grow between 3.5% and 4.5%, up from guidance between 1.5% and 2%.

On Monday, Cowen & Co. analyst Helane Becker raised her 2017 capacity growth estimate for American (AAL) - Get Report to 1.5% from 1%.

Also Monday, Delta (DAL) - Get Report announced it will begin new service between Seattle and Chicago, a route already served by three hub carriers as well as Southwest. Delta said it will operate three daily Seattle departures, starting June 19, aboard an Airbus A319 seating about 130 passengers.

In a report, issued Monday before Delta's announcement, Becker wrote, "We are seeing airlines increasing capacity and overlaying one another's capacity in the U.S. domestic market.

"We continue to view the U.S. domestic market as a zero-sum game, as the market is mature and year-over-year traffic growth is not a guarantee," she said. "When a carrier increases capacity in one market, it will directly impact the other carriers serving the same market.

"Operating margins will remain under pressure until fares start to increase," Becker said.

The issue of capacity gains is "a question that constantly comes up," John Heimlich, chief economist for A4A, the airline industry trade group, said Monday on a conference call with reporters. "I don't think it's new this year or last year.

"Carriers need to demonstrate {that} it's margin accretive capacity, that {new routes} make short-term or medium-term sense," Heimlich said. "Capacity growth in and of itself is not negative for financials. {But} it's definitely something they need to manage in how it's communicated. So far, it's been successful."

Heimlich said it's unclear whether the travel ban is impacting international travel.

"We don't have access to booking data here, {but} we are three weeks from first-quarter earnings calls -- I'm sure that will be a topic," he said.

"I suspect the strong dollar and other issues are resulting in some pockets of weakness," he said. "We've heard a lot from airlines about areas of strength -- Latin American is coming back with Brazil's economy. I suspect it's a mixed bag -- {but} I think they are more bullish than bearish at this time."

Trump has proposed a ban on the entry of travelers from six predominantly Muslim countries. It faces legal challenges and was halted earlier this month. In January, courts rejected a similar but more expansive ban.

Heimlich had been questioned about a World Travel & Tourism Council report, released Monday, that said growth in the U.S. travel and tourism sector will slow to 2.3% from 2.8% in 2016.

"Inbound to the U.S., it's the strength of the dollar, so it's much more expensive for people to go this year -- so you'll see some effect of that, and the Trump ban is not helpful," David Scowsill, CEO of the council, told CNBC on Monday. As for outbound international travel, he cited inflation and declining disposable income as barriers to growth.

Airlines including American, Delta, Copa and Qatar have said they have seen no impact from the travel ban proposal. However, Emirates President Tim Clark said recently that the January proposal led quickly to a 35% decline in Emirates' bookings to the U.S.

The ban, first implemented on Jan. 6, banned U.S. entry for travelers from Iran, Iraq, Syria, Libya, Somalia, Sudan and Yemen, all majority-Muslim countries. "The effect it had was instantaneous," Clark told reporters at a travel conference in Berlin.

"I am concerned," Clark said, according to The National, a United Arab Emirates newspaper. "It's the tone of it. We have brought millions of Muslims to the United States, but now they may not feel welcome, they may look at going on holiday elsewhere."

Editors' pick: Originally published March 21.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.