The death toll in Syria's Eastern Ghouta suburb and Idlib province has climbed to at least 180, as Syrian government and Russian forces continued their air attacks on the rebel-held areas for a fourth day.

At least 34 civilians, including 12 children and a woman, were killed in Eastern Ghouta, a rebel enclave on the outskirts of the capital, Damascus, on Wednesday, according to monitoring groups and activists.

Four others were killed in Idlib, which has been the target of recent alleged chemical attacks.

A government offensive backed by Russian air support has been under way in the area since mid-December, but both Idlib and Eastern Ghouta have come under heavy fire in the past four days after a Russian pilot was shot down and killed on Saturday.

"People here believe Russia is taking revenge after its plane was shot down by the rebels and its pilot killed," Hazem Shami, an opposition activist in Eastern Ghouta told Al Jazeera.

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"It is intense bombardment. At least 800 people are wounded. Some of them very seriously and they can't be treated here."

A search for survivors amid the rubble was under way on Wednesday by members of Syria's Civil Defensive unit.

Residents and activists have described the damage and human suffering on the ground.

"Scenes of entire buildings, housing whole families crashing down with the residents - women, children and men - still inside have become a frequent image," Abu Salem al-Shami, a resident and activist, told Al Jazeera by phone from Eastern Ghouta.

'Largest massacre'

The Syrian Observatory for Human Rights (SOHR), a UK-based monitoring group, says Tuesday marked the "largest massacre in Syria" since April's chemical attack on Khan Sheikhoun in Idlib province when more than 80 people were killed. At least 60 people were killed on Monday.

Both areas were meant to be two of several "de-escalation zones" agreed upon a year ago by Russia, Iran - both government allies - and Turkey - a backer of the armed opposition.

The deal was meant to result in an end to the violence and provide safety to civilians. But it has not been implemented.

Eastern Ghouta, which is home to 400,000 people, has been besieged by pro-government forces since 2013.

Due to the siege, very little humanitarian aid has entered, making access to basic supplies such as food, highly restricted. Medical supplies are also scarce.

Amid the attacks, doctors have put up makeshift health centres, but health professionals say they too are being hit.

"The centre is now out of service," said Beit Sawa, a doctor at Medical Centre. "It was hit by the planes. It was the only medical centre in this town and it served 15,000 people.

Many believe pro-government forces are pushing for a decisive outcome.

"Since the Syrian army is not fighting everywhere it is ready now to fight on two fronts - in the north and in east Ghouta," Hisham Jaber, a military analyst, told Al Jazeera.

President Bashar al-Assad's forces have gained ground in the southern edges of Idlib in recent weeks.

But, Al Jazeera's Zeina Khodr, reporting from Beirut in neighbouring Lebanon, said they have been "struggling to bring Eastern Ghouta under their control for years".

"Now that Russia appears to be honing in on rebel areas, Syria is once again in the midst of a major escalation."

'More than 100 government forces killed'

Meanwhile, the US-led coalition in Syria and its allied forces, reportedly killed more than 100 pro-government fighters in eastern Syria.

Quoting an unnamed US official, Reuters news agency said that the coalition was responding to an attack against the US-backed Syrian Democratic Forces (SDF) in Deir Az Zor.

"We estimate more than 100 Syrian pro-regime forces were killed while engaging SDF and coalition forces," Reuters quoted the official as saying.

The coalition confirmed on Thursday that an offensive did take place, but did not give any details.