The clashes in Syria's Idlib province in the country's northwest pit the jihadist Hayat Tahrir al-Sham (HTS) against the key rebel group Ahrar al-Sham, which is supported by Turkey and some Gulf countries.
The factions were once allies and fought alongside each other to capture most of Idlib province from the Syrian government in 2015.
But growing tensions between the two have been exacerbated, analysts say, by HTS's fears of a plan to expel the internationally designated "terror" group from the province.
The crossing was previously controlled by Ahrar al-Sham.
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"The fighting is now inside the crossing. It has become a battlefield, with part of it under Hayat Tahrir al-Sham's control, and part under Ahrar al-Sham's control," Rami Abdel Rahman, head of the Britain-based Observatory, said.
HTS also seized two villages and dispatched "huge military reinforcements" to a third to take on Ahrar al-Sham, he added.
The monitor said the fighting has so far killed at least 83 people, including 15 civilians.
HTS is dominated by the Fateh al-Sham faction, which was previously known as Al-Nusra Front before renouncing its ties to Al-Qaeda.
The clashes have been accompanied by sporadic demonstrations against HTS in several parts of the province, including in the town of Saraqeb where the jihadists on Wednesday and Thursday opened fire on protests against them.
On Wednesday, the shots killed a media activist participating in the demonstrations in the town.
The Observatory four civilians and the media activist were among the 15 civilians who have lost their lives in the violence so far.
It also reported that 68 fighters from both sides had been killed.
Experts said the outbreak of violence comes against the backdrop of a deal agreed in the Kazakh capital Astana in May for four "de-escalation zones" in Syria.
The agreement between regime allies Russia and Iran, and rebel backer Turkey, designates the Idlib region as one of the mooted zones where combat between the government and rebels would halt.