A North Atlantic right whale feeds on the surface of Cape Cod bay off the coast of Plymouth, Massachusetts, March 28, 2018. Credit: Michael Dwyer | AP

The federal government is extending the use of a protected zone off Massachusetts to help rare whales until at least later this month.

The National Oceanic and Atmospheric Administration established the zone to protect a group of right whales seen there on March 13. The agency said the zone will remain in effect through March 29.

The area is located south of Nantucket.

The agency has asked mariners to travel around the area or transit through at 10 knots or less. It said in a statement that whales have been spotted in or near shipping lanes, so mariners are asked to be “especially vigilant” in those areas.

Right whales are among the rarest large whales. They are vulnerable to ship strikes and entanglement in fishing gear.