More than 30 complaints have been received by the Scottish Environment Protection Agency (Sepa) about continued flaring at the Mossmorran chemical plant.
The regulator is taking statements from Fife residents about the impact of the gas burning, caused by a pump failure.
A formal investigation is under way into the cause of the flaring – now into its fifth day.
Operator ExxonMobil has apologised to residents.
It said it hopes to resume normal operations on Thursday.
But some have questioned whether enough is being done, with official figures suggesting incidents are increasing in regularity.
Flaring occurs to allow gas, piped from the North Sea, to be dispersed when it cannot be processed.
A bright, noisy flame can be seen as far away as Edinburgh.
One resident said the noise from the flares made it feel like they were “living by an airport runway.”
Another described it as a “total nightmare.”
A spokesman for Sepa said: “Whilst it’s important to recognise that flaring is an important safety mechanism of industrial facilities, we are disappointed at further, extended flaring by ExxonMobil.
“In response to the latest flaring incident, Sepa mobilised a full response, including air quality and noise monitoring, to assess local impacts. We’ve launched a formal investigation which will include taking statements from local residents.
“To date we have received over 30 complaints in relation to this incident and would encourage affected residents to contact Sepa’s 24-hour Pollution Hotline on 0800 807060 to record local impacts.”
ExxonMobil has been issuing daily updates on the progress of work to reduce the flaring.
Sonia Bingham, plant manager, said: “My dedicated team … continues to focus on returning this pump to service as quickly and diligently as possible.
“We are working tirelessly with our pump suppliers to achieve this, while doing everything we can to minimise the size of the flare and any disturbance to the local community.
“While nobody wants flaring, it is necessary when a production issue occurs, and is permitted by Sepa subject to strict regulation.”
Sepa said the incident was “preventable and unacceptable.”
It is understood a similar scale of investigation is under way to determine whether further action should be taken following the latest incident.