Norwegian Cruise Line announced cancellations for three ships Tuesday, adding to a growing number of called-off cruises across the industry as the omicron variant sends case numbers soaring. Citing the spread of the coronavirus and ensuing travel restrictions, major companies have canceled voyages for more than 20 ships over the past four weeks.

In addition to cancellations on eight ships, Norwegian announced last week, cruises on Norwegian Dawn, Norwegian Escape and Norwegian Joy will be scuttled until later this month. Some of the previously disclosed suspensions have also been extended.

Norwegian said it was making the changes “due to ongoing travel restrictions” and warned that itineraries on ships that do sail may change “as the global public health environment continues to rapidly evolve and destinations around the world modify their travel requirements or implement new travel restrictions.”

Norwegian’s changes this week were the latest in a string of disruptions for major cruise operators. Last week, Royal Caribbean International said it was pausing operations on three ships — Symphony of the Seas, Jewel of the Seas and Serenade of the Seas — for anywhere from a few weeks to a couple of months “as a result of the ongoing COVID-related circumstances around the world, and in an abundance of caution.” Another of its ships, Vision of the Seas, will not return to cruising with passengers until early March.

Royal Caribbean is using three of its ships to house crew who test positive for the coronavirus and need to isolate.

A handful of other cruise companies have announced more limited suspensions. MSC Magnifica, sailing in Europe, “temporarily suspended” operations last month because of travel restrictions, the company said in a statement, adding that the ship was due to restart soon.

This week, luxury liner Silversea Cruises canceled a trip leaving Fort Lauderdale on Jan. 14 due to travel restrictions in some of the port destinations.

And earlier this month, Oceania Cruises canceled South American voyages on one of its ships, Marina, for the rest of the season. Trips had been scheduled through Feb. 26, but the company said in a letter to booked passengers and travel advisers that port closures in Brazil, restrictions in Argentina and other challenges in the region forced the move.

“We understand how deeply disappointing this news is and wish to underscore that we never undertake the decision to cancel a voyage lightly,” the letter said. “Complying with the evolving testing, health, and entry requirements for Argentina and Chile has presented a significant challenge and at this juncture, with the introduction of new and prohibitive entry restrictions, we can no longer be assured of clear passage for our guests and crew.”

Cruise lines operating in Brazil — MSC Cruises and Costa Cruises — suspended operations of their five ships there at the beginning of the month until Jan. 21. The Brazilian Association of Maritime Cruises said in an announcement that it was impractical to continue sailing because authorities were applying regulations inconsistently.

The cancellations come as the highly transmissible omicron variant has caused infections to surge, both on the ground and on ships. According to the Centers for Disease Control and Prevention, cruise lines reported 5,013 coronavirus cases on ships sailing in U.S. waters between Dec. 15-29, up from 162 cases during the first two weeks of the month.

Based on that spike, the agency advised everyone — including vaccinated people — to avoid cruise travel. Cruise lines are requiring all or most people on board to be fully vaccinated, test negative before boarding and wear masks indoors unless eating or drinking. Still, the CDC increased the travel health notice for cruise travel to its highest level.

“The virus that causes COVID-19 spreads easily between people in close quarters on board ships, and the chance of getting COVID-19 on cruise ships is very high, even if you are fully vaccinated and have received a COVID-19 vaccine booster dose,” the agency said on its website.