SAN FRANCISCO : Smoke from the huge wildfire in Yosemite National Park reached the heavily-touristed heart of the park Saturday, officials said, warning visitors against strenuous activity.

Webcams showed smoke apparently clouding the world-famous Yosemite Valley, the spectacular area in the middle of the California park visited by millions every year.

More than usual visitors were expected in the park for the Labor Day weekend, although officials had said the increase could be smaller than usual due to the so-called Rim Fire, which started two weeks ago outside the park.

"Heavy smoke is now visible south of the Tioga Road, including in Yosemite Valley," said the park website's latest update after a change in wind direction from the blaze.

"Visitors to the area should avoid extended strenuous physical activities outdoors. Additionally, those (..) sensitive to air quality impacts should avoid going outside in Yosemite," it added.

Despite the warning, officials said they were optimistic of making further gains on the blaze, known as the Rim Fire, but warned that hot, dry conditions continued to create a challenging environment.

"We're hopeful that we are going to turn the corner, but it's hot, it's dry, and there is a westerly wind," US Forest Service spokeswoman Leslie Auriemmo told AFP. "There's a lot of fuel out there. We remain in a high state of alert."

According to latest figures early Saturday, the fire has burned 219,277 acres (343 square miles or 888 square kilometers) and continues to threaten 4,500 structures.

A total of 4,995 firefighters have been deployed to battle the flames, which have so far destroyed 11 homes and 97 outbuildings.

The fire, which started on August 17, was 35 percent contained as of Saturday, up from 32 percent on Friday.

Yosemite National Park officials insisted on Friday that the fire posed no threat to tourists heading to the landmark destination on a busy US holiday weekend.

The flames remain some 15 miles (24 kilometers) from Yosemite Valley, the tourist heart of the park where millions of visitors flock every year to see majestic scenery such as the Half Dome and El Capitan rock formations.

"The area where it's burning right now is mostly wilderness... There's nothing in that location that would potentially be a safety issue," said Yosemite spokeswoman Kari Cobb.

Meanwhile investigators are looking into whether an illegal marijuana farm may have triggered the blaze, US media reports said.

Several reports quoted Todd McNeal, a local fire chief in Twain Harte, one of the towns affected by the 219,000-acre (88,630 hectare) inferno, said investigators had not pinpointed the cause of the blaze.

"We don't know the exact cause," McNeal was quoted as telling a community meeting. However, he added it was "highly suspect that there might have been some sort of illicit grove, a marijuana-grow-type thing."

"We know it's human caused. There was no lightning in the area," he said.

US Forest Service officials say the cause of the fire remains under investigation.

The San Jose Mercury News reported that authorities in California have faced increasing problems with marijuana farms hidden deep in the region's rugged wilderness.