By Tim Hepher, David Shepardson

WASHINGTON (Reuters) -The head of European planemaker Airbus said novel open-rotor technology being tested by engine maker CFM International offers promising efficiencies and reductions in emissions but the "jury is out" on whether it will be commercially viable.

Chief Executive Guillaume Faury also told reporters on the sidelines of the Global Aerospace Summit, hosted by the U.S. Chamber of Commerce, that deliveries of the current generation of LEAP engines built by CFM International continue to be late.

"They are late in their deliveries," he said.

CFM had no immediate comment.

In July, the world's largest planemaker issued a profit warning and lowered its target for airplane deliveries by 30 units to 770 aircraft, blaming supply-chain problems with items including engines and cabin interiors and notably LEAP delays.

Co-owned by GE Aerospace and France's Safran, CFM is the world's largest jet engine maker by volume of engines sold. Its LEAP engines power all Boeing 737 MAX jets and about half of the rival A320neos built by Airbus, where they compete for airline contracts with Pratt & Whitney.

GE Aerospace CEO Larry Culp said the company was working closely with its top 15 suppliers where most shortages are located.

"We've made tremendous progress over the last six months going into these facilities, helping them identify and break constraints," he told the conference.

CFM, which this month celebrates its 50th anniversary, is testing a collection of technologies that could result in an open rotor engine whose main fan would be larger and more visible in order to grab more air and burn 20% less fuel.

Industry sources say Boeing is less enthusiastic about the near-term viability of aspects of the technologies in CFM's so-called RISE demonstrator, and Aviation Week recently reported that Boeing was leaning towards a more traditional engine shape.

"We're still in the process of testing the open-rotor and validating noise, vibration, performance ... We have not come to the conclusion that the open-rotor will make it. We hope it's going to be the case, because it has the potential to be by far more fuel-efficient," Faury said.

Speaking at the conference earlier, Faury reiterated Airbus aimed to launch development of a new jet by end-decade.

Pratt & Whitney and Britain's Rolls-Royce are working on designs based on geared technology with traditional engine housings or nacelles. The split raises the prospect that Airbus would be able to offer only one supplier rather than two as now.