Beirut — Intense Syrian government shelling and airstrikes on rebel-held Damascus suburbs killed nearly 130 people in two days, marking some of the deadliest bombardment of the rebellious region in three years, a monitoring group and paramedics said on Tuesday.

Retaliatory shells rained down on Damascus, killing at least eight people and causing panic among residents of the Syrian capital, seat of President Bashar Assad's power.

The violence in the capital and nearby areas came as scores of pro-government gunmen began entering the northern Kurdish enclave of Afrin. Turkish troops shelled the areas shortly after the fighters entered the area, forcing journalists to flee. Turkish state media reported the pro-government forces then fell back 10km from Afrin city.

The targeted suburbs — scattered across an area known as eastern Ghouta — have been subjected to weeks-long bombardment that has killed and wounded hundreds of people. Opposition activists say government forces have brought in more reinforcements in recent days, suggesting a major assault is imminent to recapture the area that is the last main rebel stronghold near Damascus.

The area that has been under tight siege since last year is home to some 400 000 people as well as thousands of insurgents belonging to different factions. The most powerful are the ultraconservative Army of Islam and Failaq al-Rahman with a small presence of al-Qaeda-linked fighters.

Videos from the eastern suburbs showed paramedics pulling out the injured from under the rubble while others are seen franticly digging through the debris in the dark, in search for survivors.

Monday's bombardment that killed nearly 100 people saw the use of warplanes, helicopter gunships, missiles as well as artillery, in a major escalation of violence near President Bashar Assad's seat of power.

The Britain-based Syrian Observatory for Human Rights said it was the deadliest days in eastern Ghouta since 2015, adding that 20 children and 15 women were among those killed.

The opposition-affiliated Syrian Civil Defence, also known as White Helmets, said the shelling and airstrikes killed 98 and that some people are still under the rubble. It said the dead included one of the rescue group's members, Firas Jomaa.

Both the Observatory and the White Helmets reported more airstrikes and shelling on Tuesday in eastern Ghouta as rebels pounded Damascus with mortar shells. The White Helmets reported 15 deaths on Tuesday while the Observatory said 23 people were killed.

The Observatory said the "crazy shelling" appears to be paving the way for a wide ground offensive on eastern Ghouta. If captured by government forces it would be another major victory for Assad since the conflict began seven years ago.

Rebels retaliated by hitting some Damascus neighborhoods with mortar shells, killing eight persons, including three children, and wounding 15 people, according to the state news agency SANA. On Tuesday morning, Damascus residents reported shelling of areas in central Damascus.

"Shells are falling like rain. We are hiding in the corridor," a Damascus resident told The Associated Press, asking that her name not be mentioned for her own safety. She spoke while hiding in the corridor of an office building.

"It's a bloody day," said Abdelrahman Shahin, a 31-year-old resident of Damascus who was walking on the street when a shell landed on a nearby taxi, killing the driver and a passenger in front of his eyes.

"They both died," he said, visibly shaken. "The driver was out for his livelihood, his family will be waiting for him," he said.

Syrian state TV later reported live from the streets of Damascus to show what it called the people's "steadfastness" despite the shelling. The TV reporter said residents expressed hope that the Syrian army would retake eastern Ghouta.

Army of Islam official Mohammed Alloush and Nasr al-Hariri, who heads the committee that represents the Syrian opposition, described what is happening in eastern Ghouta as a new "Holocaust" committed by the government and its backers Iran and Russia.

The UN envoy for Syria Staffan de Mistura decried a recent upsurge in violence, expressing concerns that eastern Ghouta could fall victim to widespread bloodshed like that in northern Aleppo more than a year ago. Assad's forces captured Aleppo in December 2016 in his biggest victory since the conflict began in March 2011.

"Bottom line: if we have learned something from Aleppo, (it's) time to actually avoid all this," de Mistura said.

The UN children's agency, Unicef, issued a one-page statement of protest against the killings and carried a headline, saying: "Do those inflicting the suffering still have words to justify their barbaric acts?"

The headline was followed by a blank space underneath. Unicef said it issued this blank statement because "we no longer have the words to describe children's suffering and our outrage".

The International Committee of the Red Cross also issued a statement saying that "this cannot go on". It added that "distressing reports" of dozens injured and killed every day in eastern Ghouta with "families trapped, with no safe place to hide from shelling. Dozens of mortars in Damascus cause civilian casualties and spread fear. We cannot let history repeat itself."

"The humanitarian situation of civilians in East Ghouta is spiraling out of control," said Panos Moumtzis, the UN regional humanitarian coordinator for the Syria crisis, in a statement late Monday.

"It's imperative to end this senseless human suffering now. Such targeting of innocent civilians and infrastructure must stop now," he said.

In northern Syria, Syrian state TV reported that Turkish troops shelled the entrance of the northern Kurdish enclave of Afrin shortly after scores of pro-government fighters entered the area. The TV showed shells falling in an area where journalists gathered on Tuesday afternoon forcing some of them to flee the area.

Minutes before the shelling, about 20 vehicles with heavy machine guns mounted on them were seen entering the area as part of an agreement between the government and the main Kurdish militia in Syria known as the People's Protection Units, or YPG. Turkish state media reported the pro-government forces then fell back 10km from Afrin city.

Turkey has threatened Syrian troops not to help YPG fighters, warning that it will fight its backers.

Ankara considers the YPG a "terrorist group" linked to the Kurdish insurgency within Turkey's borders. On January 20, it launched a major air and ground offensive, pounding the enclave with airstrikes and artillery on a daily basis.