At least 30,000 civilians have fled fighting between jihadists and rebels in northern Syria in the past 48 hours, Human Rights Watch says, calling on Turkey to open its border to them.

The number of Syrians trying to cross into Turkey amid an upsurge of fighting in northern Syria has nearly doubled, a Turkish official says.

The US-led coalition fighting the Islamic State group is "intensifying" its campaign in northern Syria, President Barack Obama says.