(Adds comment from sheriff about investigation into cause of fire)
By Marty Graham
SAN DIEGO, May 15 (Reuters) - A wall of flames dozens of feet high advanced on a hillside community near San Diego on Thursday, threatening to destroy upscale houses as eight other major blazes burned in Southern California, keeping thousands of people out of their homes.
Video news footage showed the rapidly moving wildfire scorching trees and brush on hilly terrain. At least one house in San Marcos, 30 miles (48 km) north of San Diego, was burned to its wooden struts and a recreational vehicle reduced to a metal shell.
Bright orange flames twisted in the wind, sending thick columns of smoke aloft and blacking out the sky. Fire engines with emergency lights flashing moved along the winding streets of the neighborhood of large Spanish-style homes.
"Even as we speak, there continue to be extraordinary acts of bravery and heroism out there on the front lines," San Diego Mayor Kevin Faulconer told a news conference.
The destructive cluster of fires comes as California enters its peak fire season in the midst of its worst drought in decades, setting the stage for what state officials worry could be a particularly intense and dangerous year.
The nine blazes raging in the San Diego environs have destroyed more than 10,000 acres (4,047 hectares), and evacuation notices have been sent to areas that are home to 125,000 residents since the first blaze broke out on Tuesday, county officials said.
"This is just the beginning of what could become the most dangerous fire season we've ever had," San Diego County Supervisor Dianne Jacob said.
It was not immediately clear how many houses might have been damaged in the latest advance of the flames toward San Marcos, but officials said the fire had already destroyed three buildings. At midday, over 12,000 people in the San Marcos area were told to evacuate, said San Diego Sheriff Bill Gore.
The blaze in the San Marcos area has raged across about 1,000 acres in steep terrain and is the top fire threat in the county, with containment at only 5 percent, officials said. No major injuries were reported as a result.
Another blaze that broke out on the Camp Pendleton Marine Base north of San Diego had charred some 6,000 acres.
Officials in the coastal city of Carlsbad said on Thursday a 400-acre (162-hectare) fire in that area was over 70 percent contained and lifted some evacuation orders. The blaze has destroyed 18 apartment units, four houses and two commercial buildings, the city said on its website.
With firefighters starting to gain the upper hand on some of the wildfires in San Diego County, authorities said they will shift their efforts to investigating how so many blazes started around the same time and whether any of them might have been intentionally set.
"We all have suspicions, like the public does, when you have nine fires that started all over the county," Gore said. "But those suspicions don't really necessarily translate into facts." (Writing by Alex Dobuzinskis; Editing by Alison Williams and Gunna Dickson)