An Islamist coalition that includes Syria's al-Qaeda affiliate al-Nusra Front has taken over one of the last government strongholds in western Idlib province, activists say.

The Jaish al-Fateh, or Conquest Army, took control of the city of Ariha after a "lightning offensive", the Syrian Observatory for Human Rights said.

However, the Syrian army said intense fighting was still going on there.

Ariha's fall would leave most of Idlib, bordering Turkey, in rebel hands.

Since late March, rebels have claimed a number of cities in the province, including Idlib and Jisr al-Shughour.

As well as bordering Turkey, Idlib is also next to Latakia province, a government stronghold and President Bashar al-Assad's heartland.

Meanwhile, the head of the al-Nusra Front said the group had been ordered by al-Qaeda's overall leader, Ayman al-Zawahiri, not to use Syria to launch attacks on the West.

In an interview with Al Jazeera, Abu Mohammed al-Julani said al-Nusra Front was focused on capturing Damascus and toppling President Assad.

He also promised to protect Syrian minorities that disavowed Mr Assad.

The hour-long interview with Julani, broadcast on Wednesday night, was his second with Qatar-based Al Jazeera since 2013, when al-Nusra Front split from what is now Islamic State (IS).

"We are only here to accomplish one mission, to fight the regime and its agents on the ground, including Hezbollah and others," he stressed, referring to the Lebanese Shia Islamist movement that is fighting alongside government forces."