• Boeing said it has extended its "best and final" offer with higher pay and other increases, but the union rebuffed it, arguing it was not a negotiated propsal.
• More than 30,000 Boeing machinists began a strike on Sept. 13 after overwhelmingly turning down a tentative agreement.
• The financial impact of Boeing's strike will depend on how long it lasts, but Bank of America estimates it's costing Boeing $50 million a day.

Boeing on Monday sweetened its contract offer and said it was its "best and final" proposal for its more than 30,000 machinists as their strike, which has halted most of the aerospace giant's aircraft production, entered its second week.

The labor union criticized the offer, saying Boeing didn't negotiate it, and called it an attempt at bypassing the union.

Boeing's new offer would boost general wages by 30% over four years, up from a previously proposed 25%. It also doubled the ratification bonus to $6,000, reinstated an annual machinist bonus and raised the company's 401(k) match.

The International Association of Machinists and Aerospace Workers District 751, the workers' union, said the new offer "was thrown at us without any discussion."

Boeing said the offer is contingent upon ratification by Friday at 11:59 p.m. PT, but a day later said it will has "reached out to the union to give them more time and offer logistical support once they decide to vote," after the IAM complained about the time crunch. The union said on Monday that Boeing "has refused to meet for further discussion; therefore, we will not be voting on the 27th."

However, it said that it will survey members about Boeing's new offer.

"We will gather your opinion on whether this offer meets your demands," it said.

After the union's response, Boeing said it had bargained in good faith with the union since formal negotiations began in March.  

The new offer is Boeing's latest attempt to end a costly strike, the unionized work group's first since 2008, as pressure is mounting on new CEO Kelly Ortberg to reach a deal.

Bank of America analyst Ron Epstein estimated the strike is costing Boeing $50 million a day, and ratings agencies have said the company risks a downgrade the longer the strike lasts.

In the first few days of the strike, Boeing said it started temporarily furloughing nonunion workers including managers, and implemented other cut costs such as a hiring freeze, reduced travel, and the elimination of first- and business-class air tickets for employees.

Both Boeing and the union said they were disappointed with negotiations last week.

"After an unsuccessful federal mediation last week, we presented a best and final offer that made significant improvements and addresses feedback from the union and our employees," Boeing said in a statement Monday. "We first presented the offer to the union and then transparently shared the details with our employees."

The strike came as workers voted 94.6% against the previous proposal that the union had endorsed.

Machinists on picket lines in Renton, Washington, told CNBC last week that they rejected the first contract with higher pay because they wanted their wages to keep up with the sharp increase in the cost of living in the Seattle area.

Some workers said in interviews that they have prepared for a long strike and have begun taking side jobs like delivering food or working in warehouses.