How do I replace XML attribute values using @xml.modify(…) with a new value stored in a @variable?

I am trying to replace the values of all attributes Items/Item/@attr inside an XML-typed variable (@xml) with a dynamically built value (@attr):

DECLARE @xml XML = '<Items>
                      <Item attr="…" />
                      <Item attr="…" />
                      <Item attr="…" />
                    </Items>';

DECLARE @newValue NVARCHAR(MAX) = …;

SET @xml.modify('replace value of (/Items/Item/@attr)[1] with "' + @newValue + '"');
--                                                             ^^^^^^^^^^^^^^^^^
--                                                         this triggers below error

However, this gives me the following error message:

Msg 8172, Level 16, State 1, Line …

The argument 1 of the xml data type method "modify" must be a string literal.

What am I doing wrong?