Concerns over proposed oil transfers in the Forth have been taken to Brussels by Mid-Scotland and Fife MSP Bruce Crawford and MEP Alyn Smith.
Contingency arrangements have been agreed for possible spills
The SNP politicians spoke with senior officials in the European Commission's environment department on Tuesday.
Mr Crawford said he was concerned because the Forth was a protected and environmentally vulnerable area.
Forth Ports' contingency plans for any accidents were recently approved by the Maritime and Coastguard Agency.
However, Mr Crawford said there was a great deal of confusion over the proposals.
He said: "Having taken Scotland's concerns to Brussels and sought advice from EU officials I am now even clearer that the Scottish government must take ultimate responsibility for the approval of ship to ship oil transfers.
"Through no fault of theirs Forth Ports faces a conflict of interest that cannot be allowed to persist.
"The commission is investigating the information they have received from the SNP and are actively taking forward a complaint."
Mr Smith said: "I welcome the commission's decision to take this investigation further and to talk to the UK authorities. It's just a shame this issue could not be sorted out in Scotland."