Packer: Packer AWS build failed with AuthFailure

I was trying to build AMI using below JSON -

{
  "variables": {
    "aws_access_key": "",
    "aws_secret_key": ""
  },
  "builders": [{
    "type": "amazon-ebs",
    "access_key": "{{user `aws_access_key`}}",
    "secret_key": "{{user `aws_secret_key`}}",
    "region": "us-east-1",
    "source_ami": "ami-9eaa1cf6",
    "instance_type": "t2.micro",
    "ssh_username": "ubuntu",
    "ami_name": "packer-example {{timestamp}}"
  }],
  "provisioners": [{
    "type": "shell",
    "inline": [
      "sleep 30",
      "sudo apt-get update",
      "sudo apt-get install -y redis-server"
    ]
  }]
}

Executing below command to build it -

packer build -var 'aws_access_key=xxxx' -var 'aws_secret_key=xxxx' /EBS/packer-templates/EC2.json

But always getting error --

==> amazon-ebs: Error querying AMI: AuthFailure: AWS was not able to validate the provided access credentials
==> amazon-ebs: status code: 401, request id: []
Build 'amazon-ebs' errored: Error querying AMI: AuthFailure: AWS was not able to validate the provided access credentials
status code: 401, request id: []

Could anyone please help why I'm getting these error? I'm providing valid access key and secret key.