Syrian rebel forces arrived at the gates of the key city of Hama on Tuesday as their fighting with the military sparked "a large wave of displacement," a war monitor said.

The Islamist-led rebels were advancing on Syria's fourth-largest city, buoyed by their lightning capture of swaths of the north in an offensive that ended four years of relative calm.

The sudden flare-up in the more than decade-old civil war in Syria drew appeals for de-escalation from across the international community.