Rebels detonated mines in tunnels under a hotel used by the army in northern Syria's Aleppo on Friday, killing at least five soldiers, a monitoring group said.
"Islamist rebels dug several tunnels under and around the Carlton hotel in Aleppo's Old City, where government troops are positioned. They used mines to detonate it this morning," said the Syrian Observatory for Human Rights.
"At least five troops were killed and 18 others wounded in the attack, which also damaged parts of the hotel," the monitoring group added.
Fierce battles broke out after the attack, killing an unknown number of rebels.
Insurgents fighting to topple President Bashar al-Assad's regime have used this tactic before, both in Aleppo, Syria's onetime commercial capital, and in Damascus province.
On another front, fresh fighting broke out between rebels and the jihadist Islamic State of Iraq and the Levant (ISIL).
Though rebels once welcomed ISIL in the fight against Assad, horrific abuses in areas under their domination have turned much of the opposition against them.
On Friday, the rebels drove out ISIL from their positions in five Aleppo provincial towns, including Hreitan, said the Observatory.
But just before their withdrawal from Hreitan, ISIL's fighters executed 13 civilians "who had links with Islamist fighters and who had been kidnapped."
Islamist and mainstream rebels opened a front against ISIL on January 3. The jihadists were expelled from the oil-rich eastern province of Deir Ezzor last week.
Elsewhere, a car bomb attack killed 18 people in front of a mosque in rebel-held Yaduda village in the southern province of Daraa, said the Observatory.
More than 136,000 people have been killed in Syria's brutal war since March 2011, and millions more have fled their homes.