Follow external command output

I know that I can right now do something like

:echo system("external --command")

and have Vim (after waiting for the command to finish) `echo` the output as a message.

Is there a way for me to have each new line of output from the command `echo`ed as a message as it occurs? This command takes some time to run and rather than wait for it to finish before seeing any output I'd like to be able to follow what it's doing as it goes.

Thanks!

Ben

--
b