symfony twig javascript function undefined

I'm trying to use an external JS file in my twig. The goal is just to verify client's input. When I put directly my script in my Transfert.html.twig , my script is well executed but when I used an external file with assetic nothing happen. The link created by assetic is good(I can see my script when i click on it in my web page source code).but firebug says

"SyntaxError: expected expression, got '<'
<script type="text/javascript">" "ReferenceError: verifyMontant is not defined"

I registred my bundle into app/config/config.yml: " bundles: [FASTTransfertBundle]", so I guess no problem form here

Now this is my code: Transfert.html.twig:

{# src/FAST/TransfertBundle/Resources/views/Default/Transfert.html.twig #}
{% extends "FASTTransfertBundle::layout.html.twig" %}
{% block title %}{{ parent() }} - Index{% endblock %}
{% block body %}
    {{ form_label(form.montant) }} {{ form_widget(form.montant,{'attr':{'onblur':'verifyMontant(this)'}}) }}
    {% javascripts '@FASTTransfertBundle/Resources/public/javascript/verifyTransfert.js' %}
        <script type="text/javascript" src="{{ asset_url }}"></script>
    {% endjavascripts %}
{% endblock %}

And this is my javascript file :

<script type="text/javascript">
//color if wrong
function changeColor(field,error)
{
    if(error)
        field.style.backgroundColor = "#fba";
    else
        field.style.backgroundColor = "";
}

function verifyMontant(field)
{
    var montant=  field.value.replace(/\D+/g,'');
    var regex = /^\-?[0-9]*\.?[0-9]+$/;
    if(!regex.test(field.value)){
        changeColor(field, true);
        return false;
    }
    else if(montant.length != 11){
        changeColor(field, true);
        return false;
    }
    else{
        changeColor(field,false);
        return true;
    }
}
</script>