Constructing a SOAP envelope using LINQ to XML

I have to construct an XML document which has a SOAP envelope like so:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://ift.tt/sVJIaE">
  <SOAP-ENV:Body>

Thought that SOAP-ENV is also a XElement, so tried like this:

XNamespace soap = "http://ift.tt/sVJIaE";
            XElement soapEnvelope = new XElement(soap + "SOAP-ENV:Envelope",
                                new XAttribute(XNamespace.Xmlns + "xmlns:SOAP-ENV", soap.NamespaceName),
                                new XElement("SOAP-ENV:Body"));

gives this error:

The ':' character, hexadecimal value 0x3A, cannot be included in a name.

Any clues?

Thanks in advance.