A QANTAS Boeing 747 flies extremely low past buildings and industrial sites on its final approach to the city's airport, in Sydney.

Travel agents and websites have begun to respond to consumer concern and the grounding of Boeing's 737 MAX planes, by changing policies and introducing options to allow customers to choose the type of aircraft they wish to fly on.

Kayak.com, part of the Booking.com stable, was the first big travel search website to say it would modify search filters to allow customers to exclude particular types of planes from queries.

Several travel agents said they were dealing with the cancellation of flights due to the grounding of nearly two-thirds of the Boeing 737 MAX planes in most countries outside North America, prompting a wave of re-bookings.

Carlson Wagonlit Travel, which manages travel for big global businesses, said some clients wished to explore the possibility of temporarily restricting travel on Boeing 737 MAX 8 planes.

The changes come after a Boeing 737 MAX 8 crashed in Ethiopia on Sunday, killing all 157 people on board and causing regulators across the world to suspend or ground the planes.

Anxious travelers took to social media to air their concerns and checked with travel agents to see if they were booked on these aircraft.

"We've recently received feedback to make Kayak's filters more granular in order to exclude particular aircraft models from search queries," a spokeswoman for the website told Reuters in an email responding to questions.

"We are releasing that enhancement this week and are committed to providing our customers with all the information they need to travel with confidence."

U.S. travel firm Expedia, Germany's Trivago and Indian online travel agents MakeMyTrip and Yatra did not immediately respond to requests for comment about the impact the crash is having on bookings.

'Phone calls keep coming'

Temporary bouts of nerves among travelers are a familiar part of the reaction to major plane crashes, and agents booking travel for major executives have remained cautious about making immediate changes to booking systems.

Norwegian travel agent Berg-Hansen, dealing with cancellations of flights on Norwegian Air's 737 MAX planes, said clients were mainly concerned with whether their flights were still scheduled to fly and the need to re-book if so.

"We have increased our staff from last night, through the night and now," Berg-Hansen Chief Executive Officer Per-Arne Villadsen said.

"Remarkably we have less phone calls than we expected, although they are more than usual. We had around 100 phone calls from midnight to 7 am this morning and they keep coming."

He said that the company was using alternative airlines including SAS, AirFrance and KLM to re-book and that he believed customers would come back to Norwegian Air if they reopened flights on the MAX in the future.

Germany's TUI, the world's largest travel services company and the operator of a holiday airline which had been flying some 737 MAX 8 planes, said its priority was to ensure that clients are able to fly.

The Sunday crash in Ethiopia was the second in the last five months involving a Boeing MAX plane. Last year, a Lion Air jet went down in Indonesia, killing 189 people.

The twin crashes have spooked the airline industry and heaped pressure on Boeing, whose shares have plunged, wiping $25 billion off its market value in the space of less than three days.