Is it possible/safe to call AWS Lambda function directly from frontend?

I have Lambda function that executes in minimum 12 seconds so I can't expose it via AWS API Gateway (because of 10 sec timeout). What I was thinking is to call/execute Lambda function directly from frontend through AWS javascript SDK. Is it safe to create IAM user that will have only permission to execute this one particular function and hardcode this user AWS_KEY_ID and AWS_SECRET_KEY in frontend?