A worker strike is costing Boeing $1 billion per month. That and other problems have driven its stock down sharply this year.

S&P Global Ratings is looking at downgrading Boeing's credit grades to junk, citing the planemaker's growing cash needs as it suffers from a protracted strike by machinists.

Last month, all three major ratings agencies including S&P had warned that a prolonged strike at Boeing's factories in the U.S. West Coast may lead to a ratings downgrade, a heada