An ExxonMobil oil tanker had a dead humpback whale on its bow when it arrived at the Valdez marine terminal, federal officials said.

It was not immediately clear whether the tanker, operated by Exxon Mobil's SeaRiver Maritime shipping unit, struck the whale or how the animal died, the US Coast Guard and National Oceanic and Atmospheric administration said.

"We don't know the condition of the whale when it was struck, or where. What we do know is it was on the tanker bow in Valdez," said Sheela McLean, an NOAA spokesperson.

"We don't have any definitive answers about it," she said, adding that she could not confirm reports that the whale was 50 feet long. Marine biologists say adult humpback whales can grow to be around 50 feet long.

The discovery was made late Monday afternoon, when the Kodiak arrived at the Valdez marine terminal of the Trans-Alaska Pipeline, officials said.

A SeaRiver Maritime spokesman said the whale was discovered just as the Kodiak was being prepared for docking, and while the ship was moving very slowly.

Neither crew members on the Kodiak nor those aboard the tugs that escorted the tanker during the final approach to Valdez noticed anything out of the ordinary during the transit, said Ray Botto, external affairs manager for SeaRiver Maritime.

"There was nothing that suggested any deviation from standard operating practice," he said, adding the poor condition of the whale, which had a noticeable stench, suggests it had been dead for a while.

The incident comes twenty years after the Exxon Valdez supertanker spilled 11 million gallons of crude oil into Alaskan waters, fouling 1,300 miles of coastline and disrupting or killing marine wildlife.