A cruise ship rammed and may have killed a humpback whale in Alaska sometime in the past few days without realizing it.

The crew of the 2,000-passenger Summit discovered the eight-metre whale pinned to the bow of the ship when it pulled into the Alaskan port of Seward on Sunday.

The ship apparently struck the whale at some point in its1,000-kilometre passage from Yakutat, at the top of the Alaska Panhandle, but the crew did not see or hear the whale.

They felt no bump during their voyage, said a spokesman for Celebrity Cruises, which owns the ship.

U.S. officials investigating the incident said the ship's operators apparently did nothing wrong.

The whale has been tentatively identified as a humpback, an endangered species. It was towed by a tugboat to a nearby beach, where it will be checked to see whether it was alive when it was struck.

The whale was on the bulbous bow that protrudes from the ship's hull below the waterline, said Scott Adams, an enforcement officer with the U.S. National Oceanic and Atmospheric Administration.

"We have no knowledge if it struck this vessel or another vessel or just died of some genetic issue," Adams said. "It doesn't look like the result of any predatory attack."

At least two humpbacks have been struck and killed by tour boats in Glacier Bay National Park in the last decade, said Barbara Mahoney, a marine mammal specialist with the National Marine Fisheries Service.

"These boats are big enough that they don't even feel a bump," Mahoney said.