* 'Rim Fire' blackens area bigger than Chicago
* Firefighters battle to stop it reaching heart of Yosemite
* Operation limits access for visitors
* Another week needed to fully enclose the blaze
Firefighters determined to keep a massive blaze from raging into the heart of California's Yosemite National Park have shut down half of its main east-west corridor, just days before a holiday weekend marking the end of the peak summer tourist season.
A long stretch of Tioga Road through the western half of the park was closed on Wednesday as fire crews tightened their grip on the blaze, extending containment lines around 30 percent of the wildfire's perimeter by the end of its 12th day.
The so-called Rim Fire has blackened about 192,500 acres (77,902 hectares), or more than 300 square miles, of dry scrub and timberlands, mostly in the Stanislaus National Forest west of the park. Its cause is still being investigated.
The firefighting force of more than 4,500 is backed by teams of bulldozers, water-dropping helicopters and airplane tankers carrying payloads of flame-retardant chemicals.
The battle inside Yosemite is focused largely on preventing flames from advancing any farther toward the core of the park, famed for its towering rock formations, waterfalls, meadows and pine forests.
Some 4 million people visit Yosemite each year, most of them during the peak months of June through August. Park officials said they have already noticed a decline in the crowds of visitors typically seen in late summer.
BIGGER THAN CHICAGO
With an overall footprint that exceeds the land mass of Chicago, the blaze ranks as the sixth-largest California wildfire on record.
As of Wednesday less than a quarter of the total burned landscape, about 43,000 acres, lay inside Yosemite, confined to the northwest corner of the 750,000-acre (300,000-hectare) park.
Earlier this week, flames closed in on the park's Hetch Hetchy Reservoir, which supplies 85 percent of the water consumed by San Francisco and several other communities some 200 miles (320 km) to the west.
But utility officials said there was little risk to the artificial lake because of the rocky terrain and lack of brush surrounding it.
On the opposite end of the sprawling fire zone west of Yosemite, crews fought to keep flames away from some 4,500 homes in a string of small communities along the fringe of the Stanislaus National Forest, said California Department of Forestry and Fire Protection spokesman Dennis Matheson.
Most of those dwellings have been ordered evacuated or were under advisories urging residents to leave voluntarily or be ready to flee at a moment's notice.
The fire has already destroyed dozens of homes and cabins, but no serious injuries have been reported.
DEFENDING YOSEMITE'S CORE
Matheson said rugged, hard-to-reach terrain was hindering efforts to carve additional fire breaks around the blaze, and he estimated it would take another week to fully enclose it.
"I think it's very safe to say that we're looking at least at the first week of September," Matheson said. "A lot of it is footwork, creating containment lines by hand."
The flames last week forced the closure of a stretch of Highway 120 that leads into the west side of the park and serves as the main gateway from the San Francisco Bay area.
On Wednesday, the closure was extended eastward along 120, also called Tioga Road, as far as Yosemite Creek midway through the park. But the rest of the road through Yosemite's only east entrance remained open, along with two other gates that allow access from other directions in the southern half of the park.
The expanded closure of Highway 120 -- the only east-west route that completely bisects the park -- was imposed to allow fire crews to establish new containment lines along the road before the blaze approaches, said Yosemite spokesman Tom Medema.
"That will limit the access for visitors to and from the east side of the park, quite possibly over Labor Day weekend, which will have a significant economic impact on the area and (be)an inconvenience for visitors," he said.
Several campgrounds and trails, along with two landmark groves of giant sequoia trees, also were closed to the public.
Firefighters also planned to burn containment lines from the Hetch Hetchy Reservoir south to Tioga Road in a bid to keep the fire from creeping further east into the park, Medema said.
The blaze has been among the fastest-moving of dozens of large wildfires raging across the drought-parched U.S. West in recent weeks, straining national firefighting resources.