By Tim Hepher, Rishav Chatterjee and Sneha Kumar

(Reuters) - European regulators have pinpointed a potentially flawed cleaning process during maintenance as investigators probe an engine fire that caused a Cathay Pacific Airbus jet carrying 348 people to turn back to base.

The European Union Aviation Safety Agency said on Thursday it was asking airlines to check the engines of more Airbus A350 jets but downgraded its response to the Sept. 2 incident, easing pressure on engine-maker Rolls-Royce.

The Airbus A350-1000 and its XWB-97 engines have been under scrutiny since the crew of a Zurich-bound flight detected an engine fire and extinguished it before dumping fuel and returning to Hong Kong, where it landed safely. The incident resulted in the grounding of a number of other Cathay flights.

Hong Kong's Air Accident Investigation Authority said on Thursday it had found a ruptured fuel hose in the jet's No. 2 engine, with five secondary hoses also showing signs of wear and tear. It said it had not yet identified the root cause.

"This serious incident illustrates the potential for fuel leaks through the ruptured secondary fuel manifold hose, which could result in engine fires," the AAIA said in a report.

Its report, unexpectedly released two weeks before the deadline for such initial summaries, started a rare exchange of contrasting safety announcements with European regulators.

The Hong Kong agency said the ongoing investigation would range from design to production, installation and maintenance and suggested the incident could have been much worse.

"If not promptly detected and addressed, this situation, along with further failures, could escalate into a more serious engine fire, potentially causing extensive damage to the aircraft," the report said.

But EASA, updating its own guidance several hours later, focused on the way such engines are maintained and called for checks on all A350 jets - including the more popular A350-900 - that have had at least two repair visits. It also downgraded its directive by dropping its previous "emergency" status, meaning it is less concerned about an immediate safety risk.

Previously it had only ordered checks on A350-1000 engines.

"In-service and in-shop inspections since then have identified that a specific cleaning process available during engine refurbishment may lead to fuel manifold main fuel hose degradation", EASA said.

One person familiar with the probe said the wrong type of cleaning product may have been used in at least one repair plant.