diff --git a/doc/mkimage.1 b/doc/mkimage.1
index 8185ff5..f9c733a 100644
--- a/doc/mkimage.1
+++ b/doc/mkimage.1
@@ -10,6 +10,9 @@ mkimage \- Generate image for U-Boot
 .RB [\fIoptions\fP] " \-f [" "image tree source file" "]" " [" "uimage file name" "]"
 
 .B mkimage
+.RB [\fIoptions\fP] " \-F [" "uimage file name" "]"
+
+.B mkimage
 .RB [\fIoptions\fP] " (legacy mode)"
 
 .SH "DESCRIPTION"
@@ -104,6 +107,13 @@ Image tree source file that describes the structure and contents of the
 FIT image.
 
 .TP
+.BI "\-F"
+Indicates that an existing FIT image should be modified. No dtc
+compilation is performed and the -f flag should not be given.
+This can be used to sign images with additional keys after initial image
+creation.
+
+.TP
 .BI "\-k [" "key_directory" "]"
 Specifies the directory containing keys to use for signing. This directory
 should contain a private key file <name>.key for use with signing and a
@@ -144,6 +154,16 @@ skipping those for which keys cannot be found. Also add a comment.
 -c "Kernel 3.8 image for production devices" kernel.itb
 .fi
 
+.P
+Update an existing FIT image, signing it with additional keys.
+Add corresponding public keys into u-boot.dtb. This will resign all images
+with keys that are available in the new directory. Images that request signing
+with unavailable keys are skipped.
+.nf
+.B mkimage -F -k /secret/signing-keys -K u-boot.dtb \\\\
+-c "Kernel 3.8 image for production devices" kernel.itb
+.fi
+
 .SH HOMEPAGE
 http://www.denx.de/wiki/U-Boot/WebHome
 .PP
diff --git a/tools/fit_image.c b/tools/fit_image.c
index b17fa2d..645e93c 100644
--- a/tools/fit_image.c
+++ b/tools/fit_image.c
@@ -124,10 +124,16 @@ static int fit_handle_file (struct mkimage_params *params)
 	}
 	sprintf (tmpfile, "%s%s", params->imagefile, MKIMAGE_TMPFILE_SUFFIX);
 
-	/* dtc -I dts -O dtb -p 500 datafile > tmpfile */
-	sprintf (cmd, "%s %s %s > %s",
-		MKIMAGE_DTC, params->dtc, params->datafile, tmpfile);
-	debug ("Trying to execute \"%s\"\n", cmd);
+	/* We either compile the source file, or use the existing FIT image */
+	if (params->datafile) {
+		/* dtc -I dts -O dtb -p 500 datafile > tmpfile */
+		snprintf(cmd, sizeof(cmd), "%s %s %s > %s",
+			 MKIMAGE_DTC, params->dtc, params->datafile, tmpfile);
+		debug("Trying to execute \"%s\"\n", cmd);
+	} else {
+		snprintf(cmd, sizeof(cmd), "cp %s %s",
+			 params->imagefile, tmpfile);
+	}
 	if (system (cmd) == -1) {
 		fprintf (stderr, "%s: system(%s) failed: %s\n",
 				params->cmdname, cmd, strerror(errno));
@@ -153,8 +159,8 @@ static int fit_handle_file (struct mkimage_params *params)
 		goto err_add_hashes;
 	}
 
-	/* add a timestamp at offset 0 i.e., root  */
-	if (fit_set_timestamp (ptr, 0, sbuf.st_mtime)) {
+	/* for first image creation, add a timestamp at offset 0 i.e., root  */
+	if (params->datafile && fit_set_timestamp(ptr, 0, sbuf.st_mtime)) {
 		fprintf (stderr, "%s: Can't add image timestamp\n",
 				params->cmdname);
 		goto err_add_timestamp;
diff --git a/tools/mkimage.c b/tools/mkimage.c
index 3760392..e2b82d0 100644
--- a/tools/mkimage.c
+++ b/tools/mkimage.c
@@ -240,12 +240,14 @@ main (int argc, char **argv)
 			case 'f':
 				if (--argc <= 0)
 					usage ();
+				params.datafile = *++argv;
+				/* no break */
+			case 'F':
 				/*
 				 * The flattened image tree (FIT) format
 				 * requires a flattened device tree image type
 				 */
 				params.type = IH_TYPE_FLATDT;
-				params.datafile = *++argv;
 				params.fflag = 1;
 				goto NXTARG;
 			case 'k':
@@ -633,14 +635,15 @@ usage ()
 			 "          -d ==> use image data from 'datafile'\n"
 			 "          -x ==> set XIP (execute in place)\n",
 		params.cmdname);
-	fprintf(stderr, "       %s [-D dtc_options] -f fit-image.its fit-image\n",
+	fprintf(stderr, "       %s [-D dtc_options] [-f fit-image.its|-F] fit-image\n",
 		params.cmdname);
 	fprintf(stderr, "          -D => set options for device tree compiler\n"
 			"          -f => input filename for FIT source\n");
 #ifdef CONFIG_FIT_SIGNATURE
 	fprintf(stderr, "Signing / verified boot options: [-k keydir] [-K dtb]\n"
 			"          -k => set directory containing private keys\n"
-			"          -K => write public keys to this .dtb file\n");
+			"          -K => write public keys to this .dtb file\n"
+			"          -F => re-sign existing FIT image\n");
 #else
 	fprintf(stderr, "Signing / verified boot not supported (CONFIG_FIT_SIGNATURE undefined)\n");
 #endif
