The U.S. government is launching an investigation into the recent deaths of endangered North Atlantic right whales.

At least 13 of the whales have been found dead this year off the coasts of Canada and New England. The whales are among the rarest marine mammals in the world and number no more than 500.

An arm of the National Oceanic and Atmospheric Administration (NOAA) said Thursday it is declaring the deaths "an unusual mortality event."

The agency says that designation triggers a "focused, expert investigation" into the cause of the deaths.

Representatives from Fisheries and Oceans Canada and NOAA Fisheries are expected to provide more information about the investigation Friday morning.

Taking precautions in Gulf of St. Lawrence

At least 10 right whales have died in the Gulf of St. Lawrence since June. Some of the whales have died due to fishing gear entanglement, but ship strikes are also suspected to be a major factor.

Transport Minister Marc Garneau announced earlier this month that ships over 20 metres in length will have a speed limit of 10 knots through an area of the gulf where whales have been spotted.

The slowdown will remain in place until the endangered whales migrate out of the areas of concern, likely in the fall, said Garneau, noting the borders of the restricted zone could change depending on the migration patterns.

Vessels that don't comply face a penalty of $6,000 to $25,000. Transport Canada and the Canadian Coast Guard will enforce the speed limit.

The federal government will also ask ships under 20 metres in length to voluntarily slow down in the relevant area.

With files from The Associated Press