The restart which caused huge unplanned flaring at the Mossmorran petrochemical plant in Fife on Thursday night "is likely to continue into the weekend".
The environmental watchdog, Sepa, said the four-hour event was a result of a problem with one of the process units and reduced capacity of ground flares.
Operator ExxonMobil apologised for any intrusion.
Many local residents took to social media to complain, with reports of the flames being visible from Edinburgh.
The plant was shut down last year following persistent flaring episodes.
A spokesman said the process of restarting the plant was "safe and controlled", adding: "Our team are working to reduce both the size and duration of the flare."
He added: "We apologise to communities for any concern that we may have caused."
In recent years, local residents have complained of repeated unscheduled flaring incidents leading to noise, disturbed sleep, light pollution and vibration to houses.
The process of flaring involves burning off gas that cannot be processed. The site, which is shared by ExxonMobil Chemical Ltd and Shell Fife NGL, is used to produce ethylene.
Although the technique is legally permitted, it causes significant light and noise pollution which the operator is obliged to minimise.
We've published an update on the causes of last night's elevated flaring, published our monitoring results and have detailed the info we have on timescales on returning to normal operations. See more at https://t.co/3pqoq6e2ZQ #Mossmorran pic.twitter.com/xxBOsuSvpL— Scottish Environment Protection Agency (@ScottishEPA) February 14, 2020
The Scottish Environment Protection Agency (Sepa) said in a post on social media that it had received a high volume of calls about the flaring.
It said that it was "working hard to address the root-causes of 'unacceptable flaring', making flaring an exception rather than the routine".
Chris Dailly, Sepa's head of environmental performance, said: "Thursday's unplanned flaring during ExxonMobil chemical's restart at Mossmorran is a real reminder of why short and medium-term solutions are critical to addressing the root causes of unacceptable flaring.
"While elevated flaring is a possibility during restart, it was not expected last night.
"Once again we heard clearly and powerfully the very real concerns and frustrations of local communities.
"The short and medium-term investment we're requiring the operators to make, from noise-reducing flare tips in 2020 and 2021 and planning, designing then delivering new ground flare capacity, will make a real difference to local communities."
Sepa later added that intial data "suggests no breach of UK Air Quality Standard".
Although the flare was not expected, Stuart Neill, external affairs manager for ExxonMobil, said the use of the flare was "completely safe and controlled at all times" to minimise the flow rate.
He said: "We recognise that the surrounding steam and light causes a dramatic visual effect, but the use of the flare was completely safe and controlled at all times.
"We appreciate that people want us to complete our work quickly. We too look forward to normal operation being restored but we hope that people understand that we cannot compromise safety as we complete the re-start."
James Glen, chairman of Mossmorran Action Group, said on Thursday the flaring was "absolutely unacceptable".
He said: "Residents feel utterly abandoned by the Scottish government and Sepa, who are letting multi-national Exxon get away with murder.
"Our Facebook page and twitter feeds have blown up. People are petrified, they can't breathe properly, they are worried sick about the pollution spewing from the plant and they have been left completely in the dark about what is going on.
"Sepa's response has been pitiful. They knew this flaring was coming but they don't even have bodies on the ground monitoring."
He added: "Ministers are in hiding and have consistently refused to come to the area to hear from local communities while ministers hobnob with Exxon and applaud their investment in the plant.
"This was investment which became inevitable until the maintenance of the plant was so neglected that two (in HSE's words) 'explosions' and 'catastrophic breakdowns' last August forced the ethylene plant to shut down for six months."
However Mr Neill accused the group of using "misinformation" to scare the public and "to further its own agenda".
Sepa later said "as well as our fixed monitoring we had regulatory and scientific staff out last night".