Git clone stuck from Jenkins pipeline

I am trying to use git commands to clone a repository within the directory of a Jenkins build
I have ssh keys stored as credentials in my Jenkns job configuration, what i try:
withCredentials([sshUserPrivateKey(credentialsId: 'my-creds', keyFileVariable: 'SSH_KEY')]) { sh 'export GIT_SSH_COMMAND="ssh -i $SSH_KEY"' sh 'git clone --verbose --progress ssh://my-host/proj/proj.git' }

Unfortunatelly it hangs at: Cloning into proj....

Any suggestions?

I know i could achieve this with the Git plugin:

git branch: 'master'
    url: 'ssh://my-host/proj/proj.git'
    credentials: 'my-creds'

But there are other operations i would like to perform after the checkout, therefore i would like to have the freedom to use git in the CLI way and not via the Jenkins plugin.

This works for a git repo on Bitbucket over SSH, but should also work on github/gitlab. my-creds being a SSH Username with Private Key credential, for which you've added the public key to your git provider.

1. Using the Git plugin

pipeline {
    agent any

    stages {
        stage('Clone Repository') {
            steps {
                withCredentials([sshUserPrivateKey(credentialsId: 'my-creds', keyFileVariable: 'SSH_KEY')]) {
                    git credentialsId: 'my-creds',
                        url: 'git@bitbucket.org:my-org/myrepo.git',
                        branch: 'master'
                }
            }
        }
    }
}

1. Using git commands. Make sure to add remote SSH host key to your known_hosts (ssh-keyscan). Otherwise git clone will fail.

pipeline {
    agent any

    stages {
        stage('Clone Repository') {
            steps {
                withCredentials([sshUserPrivateKey(credentialsId: 'my-creds', keyFileVariable: 'SSH_KEY')]) {
                    sh '''
                        ssh-keyscan -t rsa bitbucket.org >> ~/.ssh/known_hosts
                        export GIT_SSH_COMMAND="ssh -v -i $SSH_KEY"
                        git clone --verbose --progress git@bitbucket.org:my-org/myrepo.git
                    '''
                }
            }
        }
    }
}

(Instead of ssh-keyscan you can also add -o StrictHostKeyChecking=no to GIT_SSH_COMMAND, but this is not such a good idea from security perspective as it will just trust any host in this case.