ZHUHAI, China (Reuters) - China's C919 jetliner - a no-show at the country's biggest air show this week – has found it harder to meet certification and production targets amid tough U.S. export rules, according to three people with knowledge of the programme.

The state-owned manufacturer, Commercial Aircraft Corp of China (COMAC), has been unable to get timely help from suppliers and has run out of some spare parts, those people said.

As of December 2020, the U.S. has required special licenses to export parts and technology assistance to any company with ties to the Chinese military. That has thrown a monkey wrench into the C919 programme, which has been in development for 13 years - one of the longest such periods in aviation.

U.S.-linked suppliers are gradually receiving the licences, but the hiccup has slowed down Chinese certification, and months-long delays threaten to affect early production, said the people, who declined to be named because of the sensitivity of the matter.

COMAC has 815 provisional orders, but only China Eastern Airlines placed a firm order for five jets.

The state-backed airline said in August it expects to receive its first C919 by the end of the year, two in 2022 and two more in 2023.

A slow production rampup would mean the C919 will not pose a near-term threat to Airbus and Boeing, which produce dozens of narrowbodies a month.

"One of the biggest hurdles is going to be the supply chain, especially now with inflation, material availability and supplier changes," said aerospace supply chain expert Alex Krutz at U.S-based aerospace consultancy Patriot Industrial Partners.

"The suppliers may not have the liquidity to make the post-certification changes or be willing as they were a few years ago to continue supporting an initial lower-rate production programme like COMAC," he added.

COMAC is years behind its initial certification schedule - one reason it did not take the C919 to the China Airshow.

"COMAC are very preoccupied with test flights. They're behind schedule and are flying as much as they can to reach the minimum hours needed for Chinese certification," an industry source told Reuters. "Despite all the issues, COMAC is very determined to get certified, as this is a paramount political task."

Sources say that the C919 is likely to receive its type certificate from China's aviation regulator by the end of this year, but that there will be a long list of limits on flight operations. Even after the certification, COMAC must make upgrades, the sources said.