Sept 29 (Reuters) - The Federal Aviation Administration (FAA) told Boeing it has not completed work needed in order to certify the 737 MAX 7 by December, according to a letter 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.

The FAA told the company 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."

The letter 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. (Reporting by David Shepardson; Editing by Kim Coghill)