SEPA has revealed more than 700 reports have been sent to them regarding how recent flaring from Mossmorran's Ethylene Plant has affected local communities.
The environmental regulator initially received reports of elevated flaring shortly after 3:30am on Sunday.
But it adds that its four remote monitoring points around the site show there has been no breach of air quality standards.
Earlier today, ExxonMobil announced normal operations have resumed.
The company has also confirmed it's no longer using its flare.
People from across Fife reported on social media seeing it last night.
There were even sightings from Edinburgh.
Head of Environmental Performance at SEPA Chris Dailly said: "Whilst we are pleased that the site has now returned to normal operations and is no longer flaring, we remain frustrated by the frequency of flaring and the flow of information from the operator.
"We’ve also clearly heard the impact flaring continues to have on local communities through over 740 reports to us since Sunday.
"Whilst we await a full, detailed report from the operator on the cause of the latest flaring incident, the operator has indicated a compressor fault.
"The evidence we are gathering will enable us to determine whether there has been a breach of the site’s permit conditions and what our next steps should be in line with our published Enforcement Policy.
"We are clear on our expectations of ExxonMobil Chemical Limited, including requiring the installation of noise reducing flare tips followed by the installation of ground flares.
"What we need now is for ExxonMobil to step up and recognise the depth of community anger and make real progress in making flaring the exception rather than the routine - and we will employ all available measures to ensure they do so.
Meanwhile, ExxonMobil has apologised for the frustration caused to locals.
The firm insists it'll continue to work to reduce future episodes of flaring.
It says the issue was due to a faulty part on its compressor.
Plant Manager Jacob McAlister said: "Fife Ethylene Plant has now safely returned to normal operations and we are no longer using our flare.
"As previously advised, the issue was caused by a faulty part on our compressor.
"This resulted in our safety systems isolating the machine and routing gas via the flare.
"Our teams identified and replaced the part, and then conducted comprehensive checks to ensure a safe and reliable re-start of the machine.
"We will finalise our own investigation into the root cause but can confirm that, contrary to misinformation, weather was not a contributing factor.
"We absolutely understand and apologise for the frustration that the use of our flare can cause.
"We will, therefore, continue to work to reduce future occurrences.
"Thank you for your patience during this time."