At least 23 people, including seven civilians, have been killed in a car bomb attack in Syria's northwestern city of Idlib, according to a monitoring group.

The blast on Sunday evening struck the military headquarters of the "Ajnad al-Kavkaz" armed group in the city, the UK-based Syrian Observatory for Human Rights (SOHR) said.

The monitor, which gathers its information from a network of sources inside Syria, added that it was not clear whether the attack was specifically targeting the group's base.

According to the SOHR, three sisters were among the civilians who were killed in the explosion.



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There was no immediate claim of responsibility.

Persisting fighting

The rebel-held province of Idlib has seen increased violence in recent weeks as forces of Syrian President Bashar al-Assad intensify their efforts to gain control over the region.

The SOHR estimates that over the past two weeks, the Syrian army has managed to take more than 60 villages in Idlib province.

The area - particularly the city - is predominantly under the control of a former al-Qaeda affiliated group known as Hay'et Tahrir al-Sham (HTS).

Other Turkish- and US-backed armed opposition groups, however, also operate in the province.

As part of a "de-escalation" agreement signed in May 2017 by Russia, Iran and Turkey, Idlib province was meant to witness a cessation of hostilities to offer safety to the two million civilians living there.

But fighting in the area persists, leaving about half of the civilian population displaced from their homes. Thousands continue to flee as government forces press deeper into the province.

Many have also been killed in suspected Russian or Syrian government air raids. The Russian and Syrian militaries say they only target "insurgents" and deny killing civilians.

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The Syrian government also said the Idlib deal does not cover armed groups such as HTS and Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

The country has been locked in a vicious civil war since early 2011 when the government of President Bashar al-Assad cracked down on pro-democracy protests.

Since then, hundreds of thousands of people have been killed and more than 10 million have been forced from their homes, according to the United Nations.