US aircraft maker Boeing faces the risk of a credit rating downgrade one month after its labor union went on strike.

Boeing last month reached a provisional agreement with union executives on a 25-percent pay raise over a 4-year period. But union members voted it down. The walkout that started on September 13 is the first in around 16 years at the firm.

The strike involves around 33,000 employees at plants near Seattle and elsewhere.

The management later proposed a 30-percent pay hike over four years. But it couldn't close the gap with the union's demands and retracted the proposal on Tuesday.

The strike is affecting production of 777 and 767 aircraft as well as 737 Max models.

Reuters news agency quotes analysts as saying Boeing could lose over 100 million dollars in daily revenue.

Some Japanese companies are producing parts for fuselages, doors and some other components for Boeing. Concerns have been raised that they could suffer losses if the strike drags on.

S&P Global Ratings said on Tuesday that it is considering downgrading Boeing, citing fears of a possible massive cash outflow.

Boeing is believed to be rushing to improve its financial standing.