GitLab-EE: LFS S3 storage AWS credentials

Am trying to configure S3 bucket storage for LFS objects. I have the configuration in place, and have attached an IAM instance role to the server allowing S3 bucket access. However, migration fails with:

E, [2019-02-27T19:09:13.028908 #2910] ERROR – : Failed to transfer LFS object e9cc72e19ddaed9387411e7909bc875d91708bf2de652ba9cc15ad9b85b73169 with error: Missing required arguments: aws_access_key_id, aws_secret_access_key

Did not want to have to configure a user for this purpose, but appears it really wants that. Is there no other way to grant GitLab access to S3 except using access/secret keys?