Error when using StartExportTask to export RDS snapshots to S3

When calling StartExportTask API from an AWS account A for a snapshot belonging to AWS account B, I receive the following error: "An error occurred (InvalidParameterValue) when calling the StartExportTask operation: The resource xxxx is owned by a different AWS account. To export resources shared with you, first copy them to your AWS account and then export the copy.". Before calling the API, I have assumed a role with AmazonRDSFullAccess policy to allow accessing the snapshot. This error refers to the "sourceArn" parameter. However, in the AWS documentation https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_StartExportTask.html it does not mention that you can't use the API for snapshots from different AWS accounts. So, the question is, is it possible to call StartExportTask for a snapshot belonging to a different AWS account?