How do I combine url() designated css into one with gulp?

For example, if you have a/css/a.css, b/css/b.css, you want to combine them and create css/x.css.

 with reference to a/css/a.css --url(../image/A.png)
a/image/A.png
b/css/b.css --url (../image/A.png) referenced
b/image/A.png

Simply concatenating with gulp-concat disables url() references.How do I keep url() references enabled when summarized in gulp?