DAMASCUS, Oct. 30 (Xinhua) -- Syria's northern city of Aleppo is witnessing the most intense shelling and clashes since the conflict began there four years ago, following a major offensive the rebels had waged on Friday, a monitor group reported on Sunday.

Hundreds of rockets and unspecified shells targeted government-controlled areas in western Aleppo, as part of the rebels' battle of "Greater Epic of Aleppo," according to the Syrian Observatory for Human Rights.

The London-based watchdog group said the rebels of Jaish al-Fateh, or the Army of Conquest, one of 12 rebel groups waging the offensive in Aleppo, have received new advanced Grad missile and heavy bombs, whose kind is still unknown.

It said the rebels were heavily using the Grad missiles in their attacks on western Aleppo.

Following their offensive, the rebels, including ultra-radical ones, advanced into the government-controlled area of Assad Suburb in western Aleppo, in their push to break the siege imposed by the Syrian army on rebel-held areas in eastern Aleppo.

The Observatory, which says it relies on a network of activists on ground, said the Syrian army was trying to unleash a counter-offensive to recapture areas it lost in the Assad Suburb and the Jamiyet al-Zahra area in western Aleppo.

It said 55 government forces have so far been killed in the attack.

Meanwhile, tens of civilians were either wounded or killed by the violent shelling on eastern Aleppo.

The Observatory also said that Syrian soldiers suffered suffocation as a result of poisonous gas used in the battles on Sunday.

The state news agency SANA said the rebels used chemical gas in shelling the Assad Military Academy in the Assad Suburb area.

Meantime, a Syrian military source told Xinhua that the Syrian army unleashed a counter attack in Aleppo, achieving notable gains, while killing many "terrorists."

The wide-scale rebel offensive and the intensified shelling have pushed the UN special envoy to Syria, Staffan de Mistura, to express his "shock" over the shelling.

In a statement Sunday, Mistura said there were "documented reports cited from field sources" indicating tens of civilian victims in government-controlled areas in western Aleppo, including children.

"Those who claim the goal of the attack is to ease the siege on eastern Aleppo must remember that there is nothing justifying the use of random and disproportionate weapons, including heavy ones, on civilian areas, which could be tantamount to war crimes," he said.

The Syrian army has laid a siege on rebel-held areas in Aleppo in recent months, urging the rebels to surrender themselves or leave eastern Aleppo to other rebel-held areas in the northwestern province of Idlib.

The rebels didn't comply with the military repeated requests. Last week, a three-day truce, aiming to ease the evacuation of civilians and rebels who want to surrender in exchange for pardon, expired with a few civilians and rebels evacuating.

The Syrian government accused the rebels of preventing the civilians, around 250,000, from leaving.

Observers believe that Aleppo is going to be the decisive battle ground among the fighting groups, and the winner will be the one dictating its conditions to resolve the crisis, as the province contains all the groups that are supported by regional and international powers, with the civilians paying the price for this proxy war.