Carnival Corp has confirmed offloading four Holland America Line ships without naming the buyers as part of a plan to reduce its fleet by 13 vessels.

Bonheur announced buying two ships from the Carnival brand on Wednesday for its subsidiary Fred Olsen Cruise Lines, disclosing only their ages, weights and passenger capacities.

TradeWinds was able to identify them as 1,380-berth Amsterdam (built 2000) and 1,404-berth Rotterdam (built 1997) using Clarksons data.

Two days earlier, TradeWinds reported that Holland America had sold 1,350-berth Veendam (built 1996) and 1,258-berth Maasdam (built 1993) to undisclosed buyers.

Brokers said these two vessels would most likely be sold for further trading instead of scrapping.

They also revealed that Carnival's Costa Cruises brand plans to offload 1,800-berth Costa neoRomantica (built 1993) to unknown buyers.

On Thursday, Carnival said it plans to sell the Maasdam and Veendam next month, followed by the Amsterdam and Rotterdam in the autumn.

Holland America said it will cancel the four ships' remaining cruises and reassign some itineraries to other ships in the fleet.

"It's always difficult to see any ship leave the fleet, especially those that have a long and storied history with our company," Holland America Group and Carnival UK chief executive Stein Kruse said.

"However, Holland America Line has a bright future ahead that includes recent Pinnacle-Class additions, with a third sistership next year that will continue to maintain our overall capacity in the marketplace."

Holland America will notify affected passengers of the cancellations and will offer them the choice to rebook voyages on other ships in its fleet when sailing resumes, or full refunds.