A convoy of buses was deployed on Monday to transport Nusra Front militants and refugees from Lebanon's border region to Syria in exchange for Hezbollah prisoners.
Under a local ceasefire between the Lebanese Shia group and the Sunni militants, about 9,000 fighters and their relatives would leave for rebel territory in Syria on Monday, a Hezbollah media unit had said.
The Nusra Front was al-Qaeda's Syria branch until it severed ties and rebranded last year. It now spearheads the Tahrir al-Sham Islamist alliance in the Syrian war.
The deal includes the departure of all Nusra militants from Lebanon's northeast border region around Arsal town, along with any civilians in nearby refugee camps who wish to leave.
Hezbollah took most of the barren, mountainous zone of Jroud Arsal last week, after launching an offensive with the Syrian army to drive militants from their last foothold along the border.
The next phase is expected to target a nearby enclave in the hands of Islamic State.
The first stage of the ceasefire, brokered by Lebanon's internal security agency, took effect on Sunday as the two sides exchanged the bodies of fighters killed in clashes between them.
‘Buses that will transport Nusra Front militants and their families have started arriving in Jroud Arsal,’ the Hezbollah military media unit said.
The convoys came from Syrian territory and headed towards Lebanese army positions, it said.
Hezbollah has played a major role in fighting militants along the frontier during Syria's six-year war. It has sent thousands of fighters to help prop up President Bashar al-Assad's government in the battle against rebel groups.
A Lebanese security source said dozens of buses would carry militants and refugees to Syria's Idlib province, which is under the insurgents' control.
Nusra Front will release eight Hezbollah fighters under the deal, three captured in recent days and five held in Syria, the source said.
The Lebanese Red Cross has taken part in logistics.
The UN refugee agency, not involved in the deal, was trying to reach refugees in the Arsal region to evaluate whether returns were voluntary, spokeswoman Lisa Abou Khaled said.
‘UNHCR believes that conditions for refugees to return in safety and dignity are not yet in place in Syria,’ she said, as war rages on across much of the country.
The multi-sided Syrian conflict has killed hundreds of thousands of people and driven at least 11 million from their homes.
Nearly 1.5 million refugees have poured into Lebanon - around a quarter of its population - where most languish in severe poverty. Several thousand live in makeshift camps east of Arsal.