Read XML file which is referring another file using XElement

I am trying to read XML from a file like:

var XmlData = XElement.Load(@"D:\test.xml");

test.xml refers to another xml file like:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE document [ <!ENTITY systems SYSTEM 'file:D:\systems.xml'> ]>
<Root>  
    &systems;
    <Servers>
         <Server>Server1</Server>
         <Server>Server2</Server>
    </Servers>
</Root>

Contents of systems.xml

<Systems>
  <System Id="1" Name="System1" />
  <System Id="2" Name="System2" />
</Systems>

However what I get back in XmlData is

<Root>  
    <Servers>
         <Server>Server1</Server>
         <Server>Server2</Server>
    </Servers>
</Root>

How can I include the contents of systems.xml while reading test.xml