C# - Invoking Server Code After Client Code For A Control INSIDE AJAX PANEL?

I've the Javascript code to do a confirmation before deletion of some records

function confirmDelete()
{
if(confirm('Delete all?'))
{
return true;
}
else
{
return false;
}
}

I've the button code here

<asp:Button ID="btnDelete" runat="server" onClientClick="return confirmDelete();" onClick="btnDelete_click" />

If i've the button outside an update panel (basically i'm using RadAjaxPanel by Telerik) it is working fine. But when the button is inside an ajax panel, even if i click OK for deleting the records the server side code is not called.