By Tim Hepher

PARIS (Reuters) -Airbus and Rolls-Royce are preparing to brief airlines on the fallout from an engine emergency that prompted Cathay Pacific to review its fleet of A350s as pressure from airlines mounts for clarity, two people familiar with the matter said.

Barring fresh evidence as investigators examine the fuel system of a jet forced to return to Hong Kong on Monday, manufacturers have been leaning against recommending worldwide checks but the final word lies with regulators, the people said.

Briefings could go ahead as early as Thursday, they said on condition of anonymity due to the sensitivity of the matter.

Some carriers including Singapore Airlines and Japan Airlines had been conducting precautionary checks of their entire A350 fleets after Cathay Pacific said it had found 15 of its 48 A350 jets needed repairs to fuel lines.

Airbus declined comment and referred queries to Hong Kong investigators, who could not be reached. Rolls-Royce and Cathay Pacific did not immediately respond to requests for comment.

Although the stance so far suggests there is no immediate discovery of a widespread flaw, it is too early to rule out further findings or analysis that may eventually require action by other airlines, the people said, asking not to be identified.

It was not immediately clear whether the European Union Aviation Safety Agency (EASA) would endorse any decision to refrain from recommending fleet action, which threatens costly downtime as the industry wrestles with maintenance delays.

"We have no further comment at this stage as we are still completing our assessment," an EASA spokesperson said.

The A350-1000, the larger of two models in the Airbus A350 family, and its Rolls-Royce XWB-97 engines have been under the spotlight since a Zurich-bound jet was forced to return to Hong Kong after an engine problem, later traced to a fuel leak.

Initial investigations have revealed that a flexible pipe feeding a fuel injection nozzle in the XWB-97 engine was pierced, the people said.

Hong Kong investigators are now expected to focus on whether that hole was the origin of the fuel leak or was itself caused by a different problem, yet to be identified, they added.

Hong Kong's Air Accident Investigation Authority did not immediately respond to a request for comment.

Cathay Pacific said earlier it would return all Airbus A350s, including the smaller A350-900 version, to operation by Saturday following inspections and fuel line repairs on 15 jets, having cancelled dozens of flights to examine the planes.

It did not say what criteria it had used to decide whether the lines should be replaced nor whether the changes had been approved by Rolls-Royce, which typically oversees maintenance under long-term engine service contracts. Neither company responded to a request for comment on the repairs.

So far no other airline has reported damage to fuel lines. Some have carried out voluntary checks since the incident while others have said they are awaiting clarity from Rolls-Royce.

In London, shares in Rolls-Royce rose 1.8% on Wednesday, extending a partial recovery after steep falls on Monday.

(Reporting by Tim Hepher in Paris; Additional reporting by Lisa Barrington in Seoul and Joanna Plucinska in London; Editing by Franklin Paul and Jamie Freed)