By David Shepardson

(Reuters) -The Federal Aviation Administration (FAA) told Boeing it has not completed key work needed in order to certify the 737 MAX 7 by December, according to a letter from the FAA seen by Reuters.

Lirio Liu, the FAA's executive director of aviation safety, told Boeing in the Sept. 19 letter that the agency had concerns about the planemaker's submissions and sought discussion "about realistic timeframes for receiving the remaining documents."

The FAA told Boeing to turn in all remaining System Safety Assessments (SSAs) by mid-September "if the company intends to meet its project plan of completing certification work (and receiving FAA approval for this airplane) by December."

Liu said as of Sept. 15, "just under 10% of the SSAs have been accepted by the FAA and another 70% of these documents are in various stages of review and revision."

Boeing faces a December deadline to win approval from the FAA of the 737 MAX 7 and 10 variants, or it must meet new modern cockpit-alerting requirements that could significantly delay approvals.

Congress could opt to waive the requirements that were adopted as part of a certification reform bill passed after two fatal 737 MAX crashes killed 346 people and led to the best-selling plane's 20-month grounding.

The FAA added in the that "most concerning, however, is that Boeing has yet to provide an initial submittal for six of the

outstanding SSAs."

It said it expects "many of these documents will take significant time to review due to their complexity and bearing on the overall safety of the new aircraft."

"Work must be completed deliberately and in such a way that an arbitrary calendar date does not become the driving factor," Liu wrote.

Boeing said it is "discussing with policymakers the time needed to complete these certifications, following established processes."

"We are not seeking to rush this process, and believe safety is best served by allowing the 737-7 and 737-10 certification effort the time needed to complete this important work without introducing different systems," Boeing said.

"Consistent operational experience across an airplane family is an industry best practice," Boeing said.

On Sept. 15, Boeing CEO Dave Calhoun predicted the company would win approval for the smaller 737 MAX 7 this year.

Calhoun said if needed, Boeing would seek "some kind of extension" and make a case based on the "safety argument to win."

The Seattle Times reported the FAA letter earlier.

(Reporting by David Shepardson; Editing by Kim Coghill, Robert Birsel)