HOUSTON (Reuters) - The U.S. Chemical Safety Board began a probe on Wednesday into a large fire the day before at Exxon Mobil Corp's Baton Rouge, Louisiana, refinery that critically injured four people, the agency said in a statement.

A total of six people were injured in the blaze on the alkylation unit at the 502,500 barrel per day (bpd) refinery, the CSB said. Four of the injured remained in a Baton Rouge hospital burn center on Wednesday.

An Exxon spokeswoman said the company is working closely with agencies probing the blaze.

"An investigation into the cause of the incident at Baton Rouge is underway," Exxon spokeswoman Lauren Kerr said on Wednesday night.

"All appropriate agencies have been notified and we are working closely with them."

The fire broke out as work was being done to restart the alkylation unit, sources familiar with plant operations told Reuters on Tuesday. They declined to be identified as they were not authorized to speak with media.

"According to initial inquiries, flammable vapors were released during unplanned maintenance around a pump," the CSB said on Wednesday.

"Although there was no explosion, the release ignited and caused a large fire."

On Tuesday, Exxon said four people were injured in a "small fire" at the refinery.

The Chemical Safety Board has no regulatory or enforcement powers, but is tasked with determining the root causes of explosions or fires at U.S. chemical plants.

CSB Chairwoman Vanessa Sutherland said the agency had repeatedly investigated U.S. refinery fires.

"The CSB has investigated too many incidents at refineries across the country," Sutherland said in a statement on Wednesday.

"We continue to be concerned about the safety of oil and gas workers and their surrounding communities. The management of risk is an important part of any high hazard operation."

The Board also continues to work for a tougher regulatory system for safety at refineries and chemical plants, according to the statement.

(Reporting by Erwin Seba; Editing by Joseph Radford)