Islamist rebels detonated explosives planted in a tunnel under an army base in northwestern Idlib on Wednesday, killing or injuring dozens, the Syrian Observatory for Human Rights said.
The Britain-based group said the explosives blew up under a checkpoint inside Wadi Deif army base, which opposition fighters have been trying to capture for more than a year.
Video posted online showed a massive blast which hurled debris and soil dozens of metres (yards) into the air.
Observatory director Rami Abdel Rahman said there was no precise toll in the attack yet, but that "dozens of soldiers have been killed or injured."
The Islamic Front, a coalition of rebel groups, claimed the attack in a statement.
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"The heroes of the Islamic Front, in coordination with the Furqan Brigade, blew up the Wadi Deif base... after digging a 860-metre tunnel packed with 60 tonnes of explosives," it said.
Wadi Deif is the one of the regime's last remaining strongholds in Idlib province, along with the nearby Hamadiyeh military base, which is also under rebel attack.
Last week, at least 30 Syrian soldiers were killed in a similar attack involving explosives planted in a tunnel under a checkpoint near Wadi Deif.
A rebel told AFP at the time that the tunnel had taken 50 days to dig.
Opposition fighters have also used the same tactic against facilities used by regime forces in the northern province of Aleppo.