Deadly clashes erupted overnight as extremists attacked two besieged regime-controlled villages in northwestern Syria, a monitor said on Sunday.
The villages of Fuaa and Kafraya represent a tiny pocket of regime-held territory in Syria's northwest Idlib province, which is otherwise almost entirely controlled by various extremist and hardline rebels.
Occasional skirmishes erupt there but the frontline has been quiet in recent months.
Late Saturday, fighters from Al Qaeda's former Syrian affiliate attacked Fuaa and Kafraya, according to the Syrian Observatory for Human Rights.
"Hayat Tahrir Al Sham (HTS) began heavily shelling the two villages, and broke into Fuaa to attack local fighters," said Observatory chief Rami Abdel Rahman.
"This is the fiercest attack in around three years."
The fighting, which was continuing on Sunday, had killed six Syrian pro-regime fighters and at least three from HTS.
Around 10,000 people, most of them Shiite Muslims, are estimated to live in the two villages. Syrian troops also hold some territory in Idlib's east, including the key Abu Duhur airbase.
Fuaa and Kafraya are the only two places in Syria currently designated as besieged by the United Nations.
Siege tactics have been used throughout Syria's seven-year conflict, mostly by the government.
Troops have employed the tactic alongside heavy bombing to cut off food and medicine to rebel-held areas, then coerce people to agree to leave in population transfer deals. After such agreements, the United Nations no longer considers those areas as besieged.
The fate of Fuaa and Kafraya has been heavily impacted by such deals.
They made up part of the Four Towns Agreement, a deal that saw tit-for-tat evacuations and aid deliveries between Fuaa and Kafraya on one hand, and the government-besieged towns of Zabadani and Madaya.
Patients in need of medical care were also allowed to leave Fuaa and Kafraya earlier this year as part of an evacuation deal for a extremist-held part of Damascus.