Islamist militants say they have captured the Syrian city of Idlib, the second provincial capital to fall into rebel hands during the civil war of more than four years.

According to the Syrian Observatory for Human Rights, the rebels and their allies have taken full control of the north-western city.

They managed to drive back government forces in the city after several days of fierce street fighting, the British-based monitors said.

The coalition that seized Idlib city is made up of Al-Nusra Front, the official Syrian affiliate of Al-Qaeda, and several Islamist factions.

"There is still a group of soldiers fighting in the security quarter of the city, but they will not be able to reverse the situation," observatory director Rami Abdulrahman said.

State media did not report the fall of Idlib, where about 200,000 people lived before the start of the conflict.

Since then, Syrians displaced from other areas, have helped the population of Idlib to swell.

Jihadists also claimed the capture of the north-western city on official Twitter accounts.

"Thanks be to God, the city of Idlib has been liberated," they wrote.

The capture of Idlib came as UN secretary general Ban Ki-moon expressed his "anger and shame" at the world's failure to stop the conflict.

"Anger at observing the Syrian government, extremist and terrorist groups and terrorists relentlessly destroy their country," he told an Arab summit in Egypt.

"Shame at sharing in the collective failure of international and regional communities to decisively act to stop the carnage that has afflicted the Arab brothers and sisters of Syria."

More than 215,000 people have been killed since anti-government protests erupted in Syria in March 2011 and a crackdown that followed by president Bashar al-Assad.

The first provincial capital to fall was Raqqa, in the north, which was seized by rebels in March 2013.

The rebels were subsequently ousted by the Islamic State group, which has made Raqqa the de factor Syrian capital of its self-proclaimed Islamic "caliphate" in Syria and Iraq.

AFP