Armed militants detonated a tunnel rigged with explosives near the ancient citadel of Syria's northern city of Aleppo before daybreak Sunday, causing damages to the citadel's fences, an official and activists said.

The rebels in Aleppo detonated the tunnel near the citadel, in what appeared to be another bid to target government troops' positions in that area, leaving "big property losses and damages in the citadel area," according to the Syrian Observatory for Human Rights.

The London-based watchdog group said the powerful explosion was followed by intense battles between the Syrian forces and the jihadi militants amid losses on both sides.

Meanwhile, Ahmad Deeb, an official in the Syrian Directorate of Antiquities and Museums, told Xinhua that the blast left probably "slight" damages in the citadel outer fence, noting that the exact damage could not be immediately assessed due to the difficulty in reaching that area in the old quarter of Aleppo.

The citadel of Aleppo is a large medieval fortified palace in the center of the old city of Aleppo. It is considered to be one of the oldest and largest castles in the world.

Sunday's incident is the latest in a string of violence engulfing Aleppo. Several hardline jihadi groups launched last week a broad offensive to strip the government forces from their control areas in the western part of Aleppo, succeeding to storm the sprawling Scientific Research compound that constitutes the first defense line to western Aleppo.

Battles have been raging since then in that battered city.

The Observatory said the rebels fired mortar shells into government-controlled areas overnight, prompting the Syrian army to fire missiles into the rebel-held ones.

A day earlier, as many as 34 people were killed by Syrian airstrikes against the city of al-Bab, which is held by the Islamic State (IS) militants in the northern countryside of Aleppo, according to the Observatory, which said it relies on a network of activists on ground.

Aleppo, Syria's largest city and once an economic hub, has become carved out between the rebels in the eastern part of the city and the Syrian troops in the west.

The rebels attempted repeatedly to storm government areas in the hope of seizing control of the entire city of Aleppo, without a success.