BEIRUT — Airstrikes hit marketplaces in two rebel-held towns in northwestern Syria on Monday, killing at least 10 civilians and injuring dozens, a civil defense group in Syria’s Idlib province said.

The attacks marked what could be the start of a new push by Syrian government forces to retake one of the few remaining bastions held by rebels, contributing to one of the bloodiest days for Syrians in the area in months.

Hours after the Idlib attacks, an airstrike by Turkish forces occupying part of northern Syria hit the nearby Kurdish-held town of Tal Rifaat, also killing 10 civilians, mostly children, a spokesman for a Kurdish-led military group said.

The Idlib attack comes amid renewed fighting around the final rebel bastion in northwestern Syria that is crammed with civilians displaced from elsewhere in the country and hardened radical Islamist fighters. In the past week, the Syrian army has been slowly pushing into the territory, recapturing villages and inflicting dozens of casualties.

Footage released by the civilian rescue team known as the White Helmets showed pools of blood staining the ground in front of piles of onions and lettuce at a market in the town of Maaret al-Numan. Civil defense members carried bloodied bodies away from the site of the airstrike.

The group said at least nine were killed in Maaret al-Numan and one in the nearby town of Saraqeb, where a market was also struck.

Tucked away in the northwestern corner of Syria, Idlib province and some surrounding areas are the last pockets still held by the rebel factions that have fought Syrian President Bashar al-Assad’s forces since 2011.

Over the past two years, the province has become a dumping ground for unwanted combatants and civilians from other parts of the country. People were moved to Idlib aboard now-infamous green buses from other provinces once the Syrian army had retaken them.

The province was the scene of major clashes in the spring and summer. The United Nations estimated that 500 people were killed before cease-fires were brokered under Russian auspices in August.

The escalation in the hostilities threatens to trigger a new humanitarian crisis in an area already teetering on the brink of disaster. The majority of the region is controlled by Hayat Tahrir al-Sham, a coalition of extremist Islamist fighters, that has reinvented itself several times under different names in an attempt to distance itself from al-Qaeda.

Since Nov. 25, the Syrian army has been slowly advancing from the south into Idlib, retaking half a dozen villages, backed by Russian and Syrian warplanes that pound the area. Many of the civilians there have already been displaced multiple times by the fighting.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said at least 69 fighters belonging to the Syrian army, its allies and the rebels were killed over the weekend.

All of northern Syria hangs on a complex relationship between Turkey and Russia, which dictates how offensives unfold. Ankara and Moscow have struck agreements and cease-fires in the northwest of the country, as well as the northeast, where swaths of territory are controlled by Kurdish forces once allied with the United States.

Turkey and Russia agreed to work to clear the Kurds from the border after Turkey halted its own offensive in the northeast, leaving Ankara beholden to the Russians and less able to resist Moscow’s moves around Idlib.

On Monday afternoon, a Turkish airstrike on the Kurdish-held town of Tal Rifaat in northwestern Syria killed 10, eight of whom were children, and injured many more, a spokesman for the Kurdish-led Syrian Democratic Forces told The Washington Post. The strike hit a market yards away from a school and a Red Crescent center, spokesman Mervan Qamishlo added.

Tal Rifaat lies on the western outskirts of a “safe zone” that Turkey has been working to establish. Last month, Turkish President Recep Tayyip Erdogan said that, despite assurances from Washington and Moscow that forces of a Kurdish militia known as the People’s Protection Units, or YPG, have left the area, both the towns of Tal Rifaat and Manbij have yet to be cleared.

Russia is one of the Syrian government’s most powerful allies and has helped prop up Assad since it intervened militarily in Syria in 2015.

“Russia has consistently tried to bite away bits and pieces of Idlib and its environs through military pressure,” said Dareen Khalifa, a senior Syria analyst for the International Crisis Group, explaining how the province goes through regular cycles of violence.

Despite the escalations and breaks in the various cease-fires, Russia continues to assure Ankara that it has not abandoned a political process of dialogue with the rebels.