Federal officials say Boeing has completed little of the work necessary to certify new versions of its 737 Max jetliner by a year-end deadline, and the aircraft maker is lobbying Congress for more time. If the Federal Aviation Administration does not certify the planes by year end, Boeing would be required under a 2020 law to add a new type of system to alert pilots when there is a potential safety problem with the flight.

Boeing wants to avoid adding the crew-alerting system, which would further delay approval of the new Max 7 and Max 10 jets. The company argues that the system doesn’t exist on older 737s and that putting it on future Max jets would add complexity for pilots.

An FAA official said in a letter to Boeing that as of mid-September, the FAA had accepted less than 10% of Boeing’s safety assessments on the new planes, another 70% of assessments were undergoing review or changes, and in some cases, the company had failed even to submit necessary documents.

“Many of these documents will take significant time to review due to their complexity and bearing on the overall safety of the new aircraft,” the official wrote.

Boeing, which is based in Arlington, Va., said that it was working to give FAA the information it needs.