Safety concerns have been raised over new supertankers due to ship liquefied natural gas (LNG) into west Wales.
Milford Haven could become the UK's third busiest port with LNG
Newspaper reports said problems had been discovered with one vessel, leading to tests and repairs.
Campaigners in Milford Haven called for a "credible independent body" to carry out a full assessment of the ships.
Shipowners BG said safety procedures worked perfectly, and Milford port chiefs said all vessels would have to show they were "sound and safe".
Two terminals are being built at Wales' biggest port to receive LNG, which is gas cooled to liquid form to make it easier to transport.
The ships will begin to arrive by the end of 2007, and up to £4bn worth of gas is due to be fed into the UK's supply over 15 years.
Safety fears have already been expressed by some local people over the possibility of a shipping accident or leak, although Milford Haven Port Authority has assured the community it has nothing to fear.
But according to the Guardian, the latest worries relate to new ships built for the BG Group and other companies.
Delivery of liquefied natural gas could begin in October 2007
The newspaper reported that one ship delivered by a South Korean yard has been withdrawn from service and will undergo tests.
A second ship - the Gaz de France Energy - is undergoing repairs in France after problems were discovered in tests before it was handed over by the shipbuilder. The ship is not part of the BG fleet.
The Guardian also quoted the shipping industry newspaper TradeWinds reporting that faults gave concern to the owners and operators of "at least 20 other LNG tankers, either delivered or under construction".
Gordon Main, of the Milford-based Safe Haven group, said: "The point is when you are building brand new ships there may well be problems... and we are adopting a slightly experimental approach to it because there has not been an independent risk assessment."
Mr Main said he "absolutely accepted" that the LNG industry had a safe record, but its history was "pretty small".
He said the risk assessment had not been "fully comprehensive". He wanted an independent assessment, followed by a decision by planning authorities.
BG Group said the problem with the ship MKE in South Korea was "nitrogen migration in the secondary barrier above the manufacturers' guidelines".
But in a statement BG said it was not a leak from the ship, "as more alarmist commentators have incorrectly inferred".
The company said the ship's monitoring equipment detected the problem, safety procedures "worked perfectly," and there was no danger at any time.
BG said the early indications of testing were that "the issue is a result of application of the secondary barriers to the cargo tanks and not a design or material flaw".
Milford Haven Port Authority said before any LNG ship arrived it would have to show it was "in a sound and safe condition, all its certification correct and up to date, its equipment operating as it should, and any defects reported so that an appropriate assessment can be made".
The port said it had done many risk assessments and tests.
Authority officials said it was unable to refuse access to LNG ships because of the UK's open ports policy.