Firefighters battled a raging 9-alarm blaze in Chelsea early Monday morning, as old mill buildings were engulfed in flames.

The Chelsea Fire Department and firefighters from across the region responded to Forbes Industrial Park at around 12:45 a.m., for a 2-alarm fire.

“The fire quickly rose in intensity to 9 Alarms,” the Revere Fire Department posted. “The fire was in a series of old 3 to 5 story Mill Buildings, which have been vacant for many years.

“Our crews operated a Aerial Master Stream as well as multiple handlines and BlitzFire Nozzles from across the Commuter Rail tracks,” the fire department added. “Reduced access and water supply as well as multiple collapses hampered the members in fighting the fire. We always stand ready to help our neighbors!”

The 9-alarm blaze led to a huge response from across the area, including from Cambridge firefighters.

“The view from Cambridge Ladder 2’s bucket: As the sun rises over the city, elevated master streams are still operating at the Chelsea 9-alarm fire,” Cambridge Fire posted.

The massive fire was leading to delays and cancellations on the MBTA Commuter Rail.

“Newburyport/Rockport Line Update: Passengers continue to experience significant delays in both directions due to fire department activity,” the Commuter Rail posted. “Shuttle busses are operating between Chelsea & Lynn. Passengers will board a train for continued inbound/outbound service.”