Oil has been spotted on the shore after a spillage in the Milford Haven waterway.
The Port of Milford Haven said sightings have been reported in Dale and Musselwick Bay in Pembrokeshire.
It comes after Valero, which runs the oil refinery in Pembroke, reported an oil leak which is understood to have happened at around midnight on January 2/3.
Up to ten cubic metres of "heavy fuel oil" leaked from a pipe connecting the jetty to the refinery.
A clean-up operation and investigation into what happened are now underway.
Mike Ryan, Harbourmaster at the Port of Milford Haven, said oil has been seen on the shore at Dale and Musselwick Bay and that this will continue to be monitored over the next few days.
He said: “A multi-agency response is continuing at the Port of Milford Haven following an oil pollution incident at the Valero Pembroke Refinery.
"Valero have today estimated that up to 7.5-10m³ of heavy fuel oil had leaked overnight on 2/3rd January from a pipe connecting the jetty to the refinery but that by first light the leak had been contained.
"As soon as the incident was reported, just after midnight, we instigated our well-rehearsed oil pollution plan."
He said Natural Resources Wales, Pembrokeshire council, Maritime Coastguard Agency (MCA) and Valero have been involved in the clean-up operation since the early hours of January 3.
The MCA has helped the Port of Milford Haven find any surface oil and work out which way it is travelling and Mid and West Wales Fire Service has loaned the use of drone equipment.
Mr Ryan added: "The Port’s pollution vessel ‘Sea Sweep’ has patrolled and collected some surface oil; however most of the oil has naturally dispersed and the MCA report from this morning’s flight has confirmed this."
Andrea Winterton from Natural Resources Wales said: “Our priority continues to be to reduce the impact of the oil on the local environment. We have officers positioned in key locations across the Milford Haven Waterway surveying for signs of pollution washing onshore.
"The booms installed yesterday and this morning will remain in place until we are confident that the risk to wildlife and the sensitive saltmarshes has passed.
"A full investigation is underway into how the oil entered the estuary and the findings of this will be shared once concluded.”
Members of the public have been urged to avoid the oil if they discover it on the shore, particularly if they have dogs, and to call the Natural Resources Wales incident hotline on 03000 653000.
