STORY: Boeing has found more quality problems on its jets.

Reuters sources say the issues this time concern its 787 widebody planes.

They say the aerospace giant has found hundreds of incorrectly tightened fasteners on some undelivered aircraft.

The sources say the problem was discovered at plant in South Carolina, with the fasteners found to have been tightened from the wrong end.

There is no immediate concern about flight safety, but one source says Boeing is trying to understand how the problem arose - and how much work it must do to remedy the issue.

The company confirmed the checks to Reuters, and said there would be no impact on deliveries.

Even so, it's another concern for investors after a series of quality control issues at the company.

Some of its big-selling 737 MAX jets were grounded for a time earlier this year after a midair blowout on one of the planes.

The Federal Aviation Administration has blocked a plan to raise output of the model, while Boeing takes steps to ensure manufacturing quality.

Regulators said they were aware of the new concerns regarding the 787, and would work with the company to determine appropriate action.

Boeing shares are down around 30% this year amid all the problems.