Write strace output to a file

I'm trying to write the output of strace ls to a file. I know that I need to use > in order to forward output of a command to a file, but it doesn't work. It creates a file but the command prints the output of strace ls to stdout but writes the file name into the file.

$ strace ls > ls_sys.txt
...
strace output
...
$ cat ls_sys.txt
ls_sys.txt