There has been “high volume flaring” at Mossmorran after boilers failed at the Fife chemical plant.
ExxonMobil, which operates Fife Ethylene Plant at Mossmorran, apologised to communities after a huge flare could be seen from miles away.
The petrochemicals giant said a loss of steam generation meant staff at the plant would have to carry out an unplanned shutdown.
Plant manger Jacob McAlister said: “We can confirm that we have experienced mechanical failure across two of our three boilers.
“With the loss of this steam generation, we cannot continue to operate our normal processes.
“As such, we are progressing with the unit shut down to allow us to evaluate the root cause and execute necessary repairs.
“Flaring will be required while our team take the steps required to safely shut down operations.
“We will always work to minimise the timelines, and will update you as these are confirmed.
“We, again, apologise if flaring is causing any concern but reiterate that the process is safe and poses no risk to communities.”
James Glen, chairman of Mossmorran Action Group, described the flaring as “apocalyptic”.
He said: “Once again the night sky in central Fife has been lit up with apocalyptic flaring as ExxonMobil experiences yet another unplanned emergency.
“A massive plume of black smoke testified to the release of a large quantity of unburned hydrocarbons and other toxic pollutants.
“Families in neighbouring communities were again forced to suffer extensive noise intrusion, light pollution that turned night into day and consequent sleeplessness and anxiety.”
Scottish Environment Protection Agency (Sepa) said on its website: “Sepa is responding to the unplanned flaring from ExxonMobil Chemical Ltd this evening, starting Monday August 12.
“We are engaging directly with the operator to understand the cause of the incident and attending surrounding communities to assess local impacts.
“We will continue to capture air quality data from our existing particulate monitor in Lochgelly and are deploying further monitoring capabilities. Updates will be shared here on our Mossmorran hub and social media.
“Reports of pollution can be made via our online reporting tool, available 24/7, sepa.org.uk/report.”
The flaring at Fife Ethylene Plant is not related to the planned flaring at the Shell-run Fife NGL plant at Mossmorran.
Earlier this month, it was announced there would be “low intensity” flaring from the Fife NGL plant stack for three days starting on Saturday.