Fabric 2 automating deployment error when git pulling on remote server. Repository not found

I'm trying to automate my deployment with Fabric 2.

When I manually do a git pull through the command line on the remote server everything works fine.

When I try to do the same with my Fabric/Invoke script it does not allow me to pull.

It does though allow me to do git status and other commands.

The code:

# Imports
from fabric import Connection
from fabric.tasks import task
import os

# Here i pass my local passphrase:
kwargs = {'passphrase': os.environ["SSH_PASSPHRASE"]}

@task
def serverdeploy(c, branch="Staging"):
    con = Connection('myuser@myhost', connect_kwargs=kwargs)
    with con.cd("/home/user/repository/"):
        # Activate the virtual environment:
        with con.prefix("source ENV/bin/activate"):
            con.run("git pull origin {}".format(branch))

The results are:

git@bitbucket.org: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Notes:

I don't even get asked for a passphrase while doing the pull.

I have tried doing the pull without activating the environment but that didn't work either.

What could possibly be the problem?

Thanks in a advance for taking the time!