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STORY: Talks broke down between Boeing and the machinists union Tuesday.

The company withdrew a pay rise offer for striking workers, and no new talks with their union are scheduled.

Over 30,000 workers have been on strike for three weeks.

The International Association of Machinists is seeking a 40% pay rise over four years.

They also aim to restore a pension plan workers lost 10 years ago.

However on Tuesday Stephanie Pope, the head of Boeing's commercial planes business, told workers in a note that "the union did not seriously consider" the company's proposals and further negotiations "do not make sense".

More than 90% of workers voted down an offer of a 25% pay rise over four years before going on strike.

Last month, workers rejected an "improved" plan from Boeing which the company described as its "best and final", which would give a 30% pay rise and restore a performance bonus.

The plane maker - which was under pressure over its 737 MAX jet's reliability issues - is undergoing a cash squeeze.

In mid-September, Boeing furloughed tens of thousands of employees, halting production of its best-selling 737 MAX jet, and other lines.

It's on the brink of losing its prized investment-grade credit rating.

S&P on Tuesday said it placed Boeing's rating on CreditWatch negative, saying the company will likely seek incremental funding.

Analysts say Boeing needs $10-$15 billion to maintain its credit ratings.

Earlier on Tuesday, sources told Reuters Boeing is mulling a sale of stock and equity-like securities to raise billions of dollars.