PARIS (Reuters) - Rolls-Royce (>> Rolls-Royce Holding PLC), the world's No.2 aircraft engine maker, said it was too early to say whether there was a gap in the market for a new mid-sized passenger jet, which is being assessed by Boeing (>> Boeing Co) as a potential 1,000-plus plane opportunity.

Rolls-Royce's Aerospace President Tony Wood told Reuters at the Paris air show on Tuesday there was a lack of clarity around the issue and declined to be drawn on whether the company would want to provide a new engine for any such project.

Boeing said on Sunday it was exploring a potential market niche between its single-aisle 737 and wide-body 787, but had not decided whether to invest in a new plane.

Airlines have told Boeing they are interested in something about 20 percent bigger than its out-of-production 757 and able to fly further.

"There is an open question around the 757," Wood said in an interview. "There's an ongoing debate. There's certainly nothing clear yet."

Britain's Rolls-Royce currently only makes engines for wide-bodied double-aisle jets that carry over 250 passengers.

However, Wood said the focus at Rolls-Royce was on developing new technology, regardless of aircraft size.

"Our play is really one of technology and being ready for being able to put a suite of technology together to produce an engine for whichever size of aircraft actually comes," he said.

Rolls-Royce has long said it wants to develop an engine to power mid-sized planes, after it scrapped a joint venture plan to do so in 2013, but has cautioned the timing and opportunity have to be right.

That will be a matter for the FTSE-100 company's new chief executive Warren East, the former head of technology firm ARM Holdings (>> ARM Holdings plc), who takes over in July, to consider.

For now, Rolls-Royce, which competes against world No.1 aero engine maker GE (>> General Electric Company), sees the wide-bodied market is the most attractive space.

"If you look at the middle of the market ... the market has fragmented somewhat ... and it's really not clear," Wood said.

In the wide-bodied market, Rolls-Royce is benefiting from a move by some airlines to opt for bigger planes to expand capacity on routes where airport slots are constrained.

"We feel pretty confident. We've seen customers at the top end of that (mid-sized) window moving into wide-bodied aircraft," Wood said, pointing to the demand for 787s in Asia.

Nevertheless, planemakers are jostling around the middle of the market. Boeing's arch-rival Airbus (>> Airbus Group), for example, has aimed the A321neo -- the long-range version of its largest single-aisle jetliner -- to fill a gap between Boeing’s forthcoming single-aisle 737-9 MAX and the twin-aisle 787.

If Boeing decides to proceed with a replacement for the 757, a new engine will be needed, said Sash Tusa, aerospace and defence analyst at UK's Agency Partners.

"There are no engines in the likely thrust class for such a plane that are available off the shelf from GE or Rolls," he said. "For Boeing to launch it, it would likely require a new engine; the question is whether any of the engine manufacturers have the resources in either money or personnel to do this."

(Reporting by Sarah Young; Editing by Mark Potter)

By Sarah Young