scp (secure copy) to ec2 instance without password

I have an EC2 instance running (FreeBSD 9 AMI ami-8cce3fe5), and I can ssh into it using my amazon-created key file without password prompt, no problem.

However, when I want to copy a file to the instance using scp I am asked to enter a password:

scp somefile.txt -i mykey.pem root@my.ec2.id.amazonaws.com:/

Password:

Any ideas why this is happening/how it can be prevented?