RIO DE JANEIRO, June 8 (Reuters) - Commercial aircraft engine makers faced fresh pressure from airline bosses on Monday, as the companies warned that groundings and high repair costs could persist for years.

These comments were made during the International Air Transport Association's annual meeting in Rio de Janeiro.

Airline executives said engine problems remain one of the biggest obstacles facing the industry despite signs of improvement.

"There is clearly discontent, and the way to measure that discontent is the number of planes that were grounded today," LATAM Brazil CEO Jerome Caderre told Reuters, adding that his company had 12 single-aisle aircraft grounded due to engine problems.

While airline executives have long complained about engine problems, improvements in supply chains and actions taken by manufacturers such as RTX's Pratt & Whitney were expected to help keep more aircraft in service.

Some CEOs and leasing companies say there is progress.

But with the supply of engines and aircraft still limited, executives expect restrictions and obstacles to continue.

“We are certainly seeing a gradual improvement year on year,” said Andy Cronin, CEO of leasing company Avolon, in an interview. “A look at the global fleet in general reveals that it is still very limited, so we expect the pressure on the market to continue for at least another two years.”

Hundreds of A320neo aircraft, the latest version of Airbus' single-aisle planes, have been grounded worldwide, partly due to the need to wait for long periods to inspect and repair engines, and also after a manufacturing problem at Pratt affected the production of the fuel-efficient GTF engines.

Airlines have also pointed to problems with engines made by Rolls-Royce and GE Aerospace for Boeing and Airbus aircraft.

Both RTX and GE Aerospace stated that they are investing heavily to increase their engine repair and production capabilities. RTX added that it is working to improve the durability of its GTF engines.

No immediate comment was available from Rolls-Royce.

United Airlines CEO Scott Kirby told reporters on Sunday, "I think the biggest obstacle for at least the next five years will be the lack of engines."

He added, "GE is working hard. I would put them at the top of the list... Pratt has faced challenges that have received extensive media coverage, but they are sincere in their desire to resolve them and work with us... The truth is, the only company I consider to be in trouble is Rolls."

Westgate CEO Alexis von Hounsbruch said that modern engines cause higher maintenance costs, which reduces the impact of fuel savings.

He added, "There is a lot of maintenance work being done outside of the scheduled timetables... the cost of ownership is much higher than we expected."

John Rodgerson, CEO of Azul, said that some of the engines are not as durable as initially expected, adding, "They simply don't last very long."