How can I check whether user is currently using the audio seekbar?

I have an HTML audio player like this:

<audio  id="audioPlayer" controls>
    <source src="test.mp3">
</audio>

I want to display some images in sync the audio file, including when the user is moving the seekbar. However, I can't find a way to check whether user is currently using the audio seekbar.

I have tried to use the timeupdateevent with no success: the code below works only when user seeks back in time.

var audioPlayer = document.getElementById('audioPlayer');
var lastUpdateTime;

audioPlayer.addEventListener('timeupdate', function() {update();});

function update() {
    if ( audioPlayer.currentTime - lastUpdateTime < 0 )
       console.log("seeking");
    lastUpdateTime = audioPlayer.currentTime;
}

I am looking for something working on "recent" browsers (e.g. IE10+).