Amazon S3 sync to local machine failed

I'm new to AWS and I'm trying to download a bunch of files from my S3 bucket to my local machine using aws s3 sync as described in http://ift.tt/1fwddsP. I used the following command:

aws s3 sync s3://outputbucket/files/ .

I got the following error: A client error (AccessDenied) occurred when calling the ListObjects operation: Access Denied Completed 1 part(s) with ... file(s) remaining

Even though I have configured my access key ID & secret access key as described in http://ift.tt/17MIl4V

Where might the problem be?