Crews battling two dozen tenacious wildfires in drought-stricken California rushed in reinforcements Tuesday, using a window of humidity and cooler temperatures to try quench the blazes ahead of a forecast spell of drier weather.

More than 10,000 firefighters are now tackling the fires that have forced thousands to flee their homes and ravaged large tracts of land in the most populous US state.

The biggest blaze in northern California, dubbed the Rocky Fire, has drawn in 3,478 firefighters. Valiant efforts by fellow crews meanwhile had brought two other fires -- one in Shasta County and another in Los Angeles County -- under control, CAL FIRE said.

But new fires continued to pop up and quickly spread, abetted by one of the worst droughts in California on record.

"Tinder dry conditions from the drought continue to allow wildfires to burn at an explosive rate," said the California Department of Forestry and Fire Protection, or CAL FIRE.

The state is bringing in extra crews from southern California to the hard-hit north, and making firefighters already on the ground work overtime and cancel days off, CAL FIRE spokesman Daniel Berlant told MSNBC.

Resources from other states, including water-bomber planes from Colorado, were also being scrambled to the rescue, Berlant said.

In Washington, President Barack Obama was briefed on the situation in California and other parts of the west, White House spokesman Josh Earnest said, praising the bravery of the firefighters.

"These are selfless individuals and we owe them a debt of gratitude for putting their lives on the line to fight these fires and protect their fellow Americans," he said.

Many of the fires are actually made up of dozens of smaller fires started by lightning, Berlant said.

While temperatures cooled over the weekend, they are expected to rise again Wednesday, followed by more thunderstorms on Thursday, CAL FIRE said.

Thunderstorms tend to increase the threat of fires, often bringing lightning but virtually no water.

- 'Historically fast' -

"Right now, definitely conditions are dry. We are continuing to see the effects of the drought as wildfires can more easily spark and then they're going to spread at these historically fast rates," Berlant said.

The week-old Rocky Fire has forced more than 13,000 people to evacuate their homes in Colusa, Lake and Yolo counties north of Sacramento, the state's capital, where it has consumed more than 67,000 acres (27,000 hectares) of land and destroyed around two dozen homes.

Only about 20 percent of the inferno is under control, CAL FIRE said.

State-wide, at least 142,000 acres of land have burned so far.

California is in the throes of a severe drought, with much of the state completely parched and residents asked to make major cutbacks in water use.

In the northern part of the state some forests were completely engulfed by the infernos, forcing the closure of several stretches of highway.

In areas where fires had been put out, charred cars dotted some roads and trees were left smouldering on the blackened earth.

A state of emergency was issued Friday and the California National Guard has been called in, underlining the scale of the threat facing the Golden State.

A firefighter from South Dakota, Dave Ruhl, 38, was killed last week in a forest outside Alturas.