No one was hurt in the Wednesday night blaze that charred the building's doorway, damaged the lobby and burned upward toward the roof.
FBI spokesman Peter Lee said the blaze was caused by an "incendiary device," but didn't specify what it was. No bomb-making materials were found, and there were no traces of an explosion, Lee said. The FBI is leading the investigation.
Consulate workers say surveillance footage showed a person coming out of a van parked outside the compound with two buckets, pouring the contents on the front of the building and setting it on fire. The consulate has a surveillance camera outside the building located in San Francisco's Western Addition district.
"We strongly condemn this despicable act and have already made representations with the U.S. on the attack," said Wang Chuan, a spokesman for the Chinese consulate in San Francisco. "And we hope that the U.S. takes all necessary measures to provide adequate protection to the consular personnel and properties and bring the culprits to justice as soon as possible."
The consulate is awaiting results from the investigation and will not speculate on who is behind the attack, Chuan said. He said they don't yet know how much repairs will cost.
Lee said he did not know whether the consulate had received any threats recently or any demonstrations at the site. Police stepped up their presence in the area around the consulate building on Thursday.
The U.S. State was in immediate contact with Chinese consulate and embassy in Washington after the fire, department spokeswoman Marie Harf said.
Harf said it was too early to judge whether security at the consulate had been adequate.
"We take this incident very seriously, and the Bureau of Diplomatic Security is working with the FBI and local authorities to investigate and apprehend the perpetrators," Harf said. "Department officials are communicating with Chinese officials to provide support and updates on the case."
Police and firefighters arrived at the scene of the fire and crews brought the flames under control within minutes, fire department spokeswoman Mindy Talmadge said.
The fire caused "serious damage to the facilities of the consulate and endangered the safety of the consulate officials and the citizens living nearby," Chuan said.
Associated Press writer Matthew Pennington in Washington, D.C. contributed to this report.
Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.