BEIRUT — Syrian rebels linked to al-Qaida took control Monday of a key border crossing connecting Turkey to a Syrian province considered the heart of support for the government of President Bashar Assad, in heavy fighting that took the lives of three members of Assad’s family.

Syrian state television accused both the rebels and Turkey of using the Turkish side of the Kasab border crossing to ambush government forces on Friday. That sparked a broader confrontation that saw both sides heavily reinforce their positions over the weekend.

The offensive has been led by the Nusra Front, al-Qaida’s official Syrian franchise, and the Islamic Front, a broad coalition of Islamist fighters that includes the al-Qaida-linked group Ahrar al-Sham. Adding to the radical flavor of the fighting is a unit of Chechen-led fighters aligned with the rogue Islamic State of Iraq and Syria, a jihadist group made up largely of non-Syrians that has been criticized for its brutal rule in the parts of Syria it controls.

On Monday, the official social media accounts of the Islamic Front released photos of its fighters taking control of the border crossing, although reports said that fighting for control of the area was continuing.

The crossing provides access to the coastal province of Latakia, the ancestral home of the Assad family and its fellow Alawite Muslims, where support is strong for the Assad government and its militia, the National Defense Force, a paramilitary group trained by Iran to bolster the thinly spread Syrian army. Latakia has both symbolic and strategic importance, connecting the rest of Syria to the Mediterranean Sea. Russia maintains a military facility at the Latakia port city of Tartus.

The importance of the region to the Assad regime is reflected in the command structure of the National Defense Force there. On Sunday, Syrian state television announced the death in the fighting of the local defense force commander, Hilal Assad, a distant cousin of the president.

On Monday, Syria announced the deaths of two other cousins of the president, Ali and Kifah Assad, though it was unclear if they were killed separately of Hilal, who died when rockets fired by rebels targeted his command post. Both men were rumored to have been in command positions in the National Defense Force and could have been killed in the same incident.

A group that is part of the Islamic Front, Jaysh al-Islam, issued a statement detailing how Hilal Assad was killed.

“The first rocket was fired around 7:15 p.m., followed by another five minutes later,” the statement said. “The rockets targeted a house where Hilal was holding a meeting with other members of the National Defense Force.”

The Syrian charge that Turkey was helping the Islamists came one day after Turkish jets downed a Syrian fighter-bomber as it attacked rebel positions along the border.

• MCT Campus