WATCH: wildfire in California's Yosemite National Park doubles in size

A wildfire threatening the largest grove of giant sequoias in Yosemite National Park more than doubled in size in a day, and firefighters were working in difficult terrain Sunday to protect the iconic trees and a small mountain town as the U.S. weathers another very active year for fires.

Campers and residents near the blaze were evacuated but the rest of the sprawling park in California remained open, though heavy smoke obscured scenic vistas and created unhealthy air quality.

The cause of the Washburn Fire was under investigation.

It had grown to nearly 2.5 square miles (6.7 square kilometers) by Sunday morning, with no containment.

Also watch: Europe heatwave: Firefighters in Spain struggle to contain wildfires

The blaze is near the iconic grove of about 500 mature sequoias, which are the world’s biggest trees by volume.