A new passenger jet being developed by Mitsubishi Aircraft Corp. that left for the United States on Saturday for flight tests was forced to return due to problems with the air-conditioning system, the company said.

The Mitsubishi Regional Jet, which has been mired in a series of delays during its development, left the city of Nagoya at around 11:50 a.m. for its first stop in Hokkaido — only to return an hour later.

The aircraft had been expected to arrive at Grant County International Airport in the state of Washington on Monday following refueling stops in Russia and Alaska, according to Mitsubishi Aircraft, a subsidiary of Mitsubishi Heavy Industries Ltd.

The flight has been postponed until Sunday or later.

A total of 2,500 hours of flight tests are required for the MRJ to obtain safety certification. The company plans to conduct most of the flights in the United States, as Grant County is known for sunny days throughout the year.

The development of the MRJ, the country's first domestically produced jet, has suffered delays due to changes in the design, manufacturing process and parts.

The jetliner for short- to medium-haul flights made its maiden flight last November.

However, the first delivery to ANA Holdings Inc. has been pushed back by around a year until mid-2018.