How to implement incremental folder synchronization from ftp1 to ftp2?

Question

Backup

Greetings.
How to implement incremental folder synchronization from ftp1 to ftp2?
The command is used via crontab:
/usr/bin/lftp -e 'mirror --only-newer --log=/var/logs/lftp.log --no-umask /folder_images/ /var/www/sitename/folder_images/; bye;' -uLOGIN,'*******' ftpname.ru
Despite the "--only-newer" option, the folder_images folder is fully synchronized every time it is started, despite the fact that the changes affected, at most, 10-20 files out of several dozen thousand.
How to setup lftp?
Vladimir.