Some three hours ago an air- or missile strike in Syria's Idleb governorate hit a meeting of leaders of the al-Qaeda aligned Haras-al-Din and Hay’at Tahrir al-Sham (HTS) aka Jabhat al-Nusra. Both were killed. It is likely that leaders of other Jihadist groups were also present. The hit completely destroyed a Haras al-Din guesthouse or headquarter. The Syrian Observatory says that more than 40 people were killed in the strike. The hit will make it much easier for the Syrian army campaign to liberate Idleb governorate.

Idleb governorate - Red=Syrian army, Green=Jihadis, Yello-Kurds

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Since yesterday evening a unilateral ceasefire was held up by the Syrian and Russian forces. Today none of their jets bombed targets in Idleb governorate. Whoever committed today's strike must have coordinated that with the Russian command beforehand.

The Observatory notes (machine translated):

The Syrian Observatory for Human Rights documented the killing of more than 40 people from jihadi groups, including leaders, as a result of rocket attacks on the headquarters and meeting room of the “Haradh al-Mu'minin” operation, whose guards are the main pillars in addition to other jihadi groups, in the area between Kefraya and Maarat Masrin east of Idlib The strikes were the result of aerial bombardment by international coalition aircraft, and the Syrian Observatory has just published that it rocked more than 7 fierce explosions in the area between the city of Idlib and the town of Kefraya east of the city on Saturday afternoon, 31 August, caused by rocket strikes targeting Headquarters for the guards faction It is not known whether the strikes were carried out by international coalition aircraft over Turkish territory, or whether they were long-range missiles ...

The impact of the strikes was quite impressive:

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Videos shows that the target was a two story house and some side buildings in the middle of an orchard. The house pancaked and is completely destroyed. The so called 'White Helmets' swarm all over the place. One wounded child is visible in the first video.

The strike might have been done by either the U.S. or by Turkey. It is likely Turkey that has the better on-the-ground intelligence insight on these groups as it has for years directly supplied them with weapons, munitions and other goods. The Turkish President Erdogan changed his position on Idleb after his recent visit to Moscow. He stopped the supplies to the 'rebels' to allow the Syrian campaign against them to succeed. The so called 'rebels' it has long supported now turn against it.

Another data point that confirms that take is a sudden lack of anti-armor missiles in the hand of the 'rebels':

The use of anti-tank guided missiles (ATGM) by militant groups against the Syrian Army in Idlib province has fallen to almost zero as of the final days of August. 
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Since the Syrian Army kicked-off its Greater Idlib ground offensive against Islamist groups in early May, about 100 guided missiles from various systems have been used by militant groups during the three and a half months since.

It is pretty obvious that Turkey ended its missile supply to the 'rebels':

[O]ne only needs to assess the current attitude on pro-militant social media right now – an attitude which is highly critical of Turkey’s lack of intervention in Idlib, a major part of this lack of intervention being identified by these sources as the sudden “collapse” (or rather, withholding) of ATGM supplies by Turkey.

The cutting off of supplies for the Jihadists in Idleb and the closing of the borders to 'rebels' who want to flee to Turkey made the country a target for those fighters. The Turkish president may well have calculated that killing their leaders now will lessen the danger of getting attacked by them.

Abu Muhammad al-Joulani, the leader of HTS, had led several campaigns against Syrian government forces.

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It was Joulani who had the command over all Jihadis and 'rebels' in Idleb in their fight against the Syrian army.

Haras-al-Din has sworn allegiance to Al-Qaeda leader Aywan al-Zawahiri. Joulani, the leader of HTS/Nusra, claimed that he had ended his relation with al-Qaeda but he never publicly retracted the loyalty oath he had given to al-Zawahiri.

The death of the leaders of these groups will not end the existence of these entities. History shows that leader assassinations tend to bring  more competent and more brutal leaders to the fore. But for at least a short while there will be more discussions about the new leaders and directions into which the groups will move than preparations on the front lines. This distraction can be used by the Syrian army to progress in its campaign.

The Syrian army is preparing to liberate Idleb governorate starting from the south with the most likely next target being Maarrat al-Nu'man. There is also a buildup of tanks and troops west of Aleppo city. The general attack axis of both fronts will run along the M5 motorway. In the west a third front will run along the M4 highway with the first main target being the city of Jisr ash-Shhugur. The front there is more difficult because the area is mountainous and held by Turkman and Uyghur fighters who still seem to have Turkish support.

The current unilateral ceasefire, which I presume was only announced to allow for today's hit, will end again soon. It was likely the last one before the real big fight to liberate Idleb governorate begins. As the 'rebels' and Jihadis no longer have access to fresh supplies the speed and the progress of the campaign will likely be impressive.

Update (16:40 UTC): The U.S. claims to have carried out the strike.