The rebels are forces opposing President Bashar al-Assad and an offensive launched by the Syrian army a year ago.
He said that this is undermining efforts to reach a political solution to the war in that country.
Two auto bomb explosions have taken lives of dozens in Syrian Idlib province, local media reported.
The semi-rural area east of the Syrian capital is home to around 400,000 inhabitants and is targeted nearly daily by regime forces trying to flush out rebel groups.
An alliance dominated by Fatah al-Sham controls much of Idlib province where there are regular auto bombings, often blamed on disputes between armed factions.
Although the long-term success of Syrian government forces looks inevitable, it will take them time to re-establish central control.
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Israel has carried out dozens of air strikes in Syria in the course of Syria's civil war, against what it says are suspected arms shipments believed to be bound for Lebanon's Hezbollah militant group, which is fighting alongside Syrian government forces.
Another pocket where rebels are still holding out however is Eastern Ghouta, a semi-rural area east of the capital Damascus that is home to some 400,000 people. The base could provide President Bashar al-Assad's forces with an important step towards reclaiming the Idlib province.
The Idlib Media Center activist group posted a video online of trucks carrying what appeared to be furniture and clothes that it said belonging to Idlib's displaced people.
According to the Observatory, the fighting in Harasta since the base was surrounded in late December left 72 regime fighters and 87 rebels dead.
The nature of the explosion was not immediately clear.
As part of a "de-escalation" agreement signed in May 2017 by Russia, Iran and Turkey, Idlib province was meant to witness a cessation of hostilities to offer safety to the two million civilians living there. Meanwhile, Tahrir al-Sham Front stated that regime's operation in Idlib "is no picnic".